The series consists of four books, and introduces three new monsters in each one:
- 1: Monster of the Month Club (1994)
- Featured Monsters: Icicle (January), Sweetie Pie (February), Shamrock (March).
- 2: Monsters in the Attic (1995)
- Featured Monsters: Chelsea (April), Burly (May), Summer (June).
- 3: Monsters in Cyberspace (1997)
- Featured Monsters: Sparkler (July), Butterscotch (August), Owl (September).
- 4: Monsters and My One True Love (1998)
- Featured Monsters: Goblin (October), Cranberry (November), Bow (December).
Not to be confused with Monsters in My Mailbox, a picture book with a very similar plot.
This series provides examples of:
- Affectionate Nickname:
- Mr. Tamerow calls Rilla "Rilly".
- Book 3 reveals that Rilla's mother was nicknamed "Sparrow" by Rilla's father, which is why she chose it for her legal name when Rilla was three.
- Alpha Bitch: Tina Welter is a pre-teen version of this, and isn't shy about mocking Rilla about her family's lifestyle and anything else she can think of. She mellows out a lot between books 3 and 4 though.
- Amazingly Embarrassing Parents: Rilla is very embarrassed by her mother and aunt's behavior, especially around her friends and part-time classmates.
- Big Eater: The monsters tend to be this — at one point in book 1, Rilla informs Icicle that the food he'd eaten the night before was supposed to last a week.
- Birthday Episode: The last five chapters (18-22) of book 1 serve as this, covering Rilla's thirteenth birthday and how she finally learns the truth about where the monsters are coming from.
- Bookworm: Icicle, the January Selection, spends most of his time reading in his corner of Rilla's room, usually books with monsters in them (such as Beowulf, Frankenstein and Where the Wild Things Are). Sweetie Pie largely prefers to be read to instead (her favorite book is Pinkerton).
- Brought Down to Normal: When Rilla first meets Owl, he can absorb knowledge and correctly use it from the paper he eats with things printed on it, but he loses the ability permanently after his glasses are removed once. Even putting them back doesn't undo it, nor does his being de-animated and later reanimated by stellar alignments.
- Calling Parents by Their Name: Sparrow insists on this from Rilla, "So Daughter would see Mother as Friend and not Authority Figure" (her justification for it). She also wants Rilla to not refer to her Aunt Poppy as "Aunt", but Rilla insists because it's what she considers the normal thing to do.
- Calling the Old Man Out: Woman, in this case, since Rilla has it out with her mother in book 2 when she finally gets tired and frustrated with her over a variety of things, including Sparrow embarrassing her in front of her friends. Sparrow is somewhat taken aback, but she and Rilla do sit down and talk over things as a result, including asking Rilla to tell her if Sparrow is embarrassing her so she'll stop doing it.
- Christmas Elves: Bow, the December Selection, is based on one.
- Competition Freak: Burly, the May Selection, is a real sports nut. His card states that he "Doesn't like to lose".
- Creepy Good: Goblin, the October Selection. A trenchcoat-wearing werewolf-like monster, she gives Rilla the creeps, but is good at heart.
- Cute Monster: The twelve monsters are all small creatures that could easily pass for stuffed animals.
- Disappeared Dad: Rilla's father, David Charles Pinowski, who split up from her mother before Rilla was ever born. Part of the plot of book 3 is her using the internet to try and find him, which she ultimately succeeds at, allowing him to visit for Christmas in book 4. She also learns he and Sparrow had tried to find one another, but he was in the Peace Corp at the time and never got her letters, and she'd changed her name by the time he tried to find her.
- Divorce Is Temporary: Subverted in book 4. Rilla would love to see her parents back together, but Sparrow tells her that that isn't going to happen, because they've just grown apart and because Rilla's father is already seeing someone back in Oregon.
- Edible Theme Naming: Rilla privately names the cats in the barn after sweet food and drink — the mama cat is Oreo, and the kittens are Pepsi, Dorito and Milk Dud (who gets renamed Milky by Tina when she adopts him in book 4). The theme continues with the dog Taco, who joins them in book 2.
- Extra Eyes: Icicle has seven eyes, all on his face. Averted by the other eleven monsters, who just have two apiece.
- Extreme Omni-Goat: Book 1 mentions that the B&B's original lawnmower was a goat named Nancy, but she had to relocate to a farm outside of town after escaping and eating things in other peoples' yards (along with making one family move because of her odor).
- Full-Name Ultimatum: Rilla's mom and aunt do this to her on occasion when she's in running late, or doing something they don't approve of.
- Gender-Equal Ensemble: The twelve monsters, when brought together. The six from odd-numbered months are all male and the six from even-numbered months are all female.
- Girls Love Stuffed Animals: Rilla has a massive collection. In fact, that's how she got signed up for the Monster of the Month Club — her membership was a present from someone who knew about her love of stuffed animals and, when they found out a company was starting a line of monster-based plushes, thought she'd like it.
- Granola Girl:
- Rilla's mother is a real New Age "back-to-nature" type with strong beliefs about environmentalism and how it's wrong to keep or name animals, and is trying to raise her daughter to be the same. Rilla, however, only goes along with her mother's methods where Sparrow can see, and privately rebels when she can, such as eating junk food when she's away from home and naming the cats in the barn after sweet food and drink.
- Rilla's aunt Poppy is a lesser version — she largely goes along with Sparrow's methods, but openly adores the mechanical riding lawnmower they received (and the feeling of power it gives her) after they had to give up the goat Sparrow had originally used to keep the lawn short.
- Butterscotch, the August Selection, is based on one; she also likes the Peace Sign pendant Rilla has from her father and ends up claiming and wearing it herself.
- Hates Being Alone: "Doesn't like to be alone" is a trait noted on the card that came with Sweetie Pie, the February Selection, and she proves it by not wanting Rilla to leave her in the attic (apparently she doesn't consider Icicle as company).
- Homeschooled Kids: Main protagonist Rilla Harmony Earth started being home-schooled because of her mother's beliefs (Sparrow Earth is a "back-to-nature" type). There's also a group of six kids (Andrew, Josh, Kelly, Marcia, Tina and Wally) who are community homeschooled; Rilla joins them for field trips and special events. When the new school year starts in book 3, it's revealed that they voted to get together more often the previous year.
- I Just Want to Be Normal: Rilla is more of a case of "I just want my family to be normal and not weird environmentally-obsessed people". Though she does have moments where she isn't too fond of having to care for monsters all the time.
- I'm Taking Her Home with Me!:
- Burly brings a dog home with him in book 2 as a pet after his night out. Luckily, he was already a stray, so Rilla's able to keep Taco even after Burly stops being alive.
- Tina really wants to do this with the kitten Milk Dud in book 4. Rilla finally agrees to it, since she sees that Tina really cares for him.
- Innocently Insensitive: When Owl uses Rilla's e-mail to contact people with "monster" in their screennames and ask if they're a real monster, he ends up offending pretty much all of them, but doesn't seem to understand why they're insulted.
- It's All About Me: Many of the monsters are rather self-centered. Icicle in particular is very egotistical, borrowing Rilla's things without asking and basically acting as if he's King of the Attic.
- Love Triangle: Rilla has a not-so-secret crush on her friend Joshua Banks. So does their classmate Tina Welter, which is one of the reasons she and Rilla don't get along. Josh, for his part, is largely content to just be friends with them both.
- The Magic Goes Away: By the end of each book, the stars have passed out of alignment for a time, causing Rilla's latest monster companions to revert to being stuffed animals, though it's hinted they may not be entirely inanimate. They also all awaken for the duration of the eclipse in book 4, but revert to stuffed toys when it ends.
- Meaningful Rename: Sparrow Earth thinks of her, her sister's and her daughter's respective renamings as meaningful, to represent the family's lifestyle as "children of the Earth". Rilla's less convinced.
- Sparrow Harmony Earth was born Donna Knox, went by Donna Pinowski when she was married, and returned to her maiden name after they split up. Book 3 reveals that "Sparrow" was her husband's nickname for her, because she was rather flighty and reminded him of a sparrow in flight.
- Poppy Harmony Earth was, after a string of marriages that each ended in turn, Sally Knox Bailey Hailey Hobbs Street.
- Rilla Harmony Earth used to be Rilla Pinowski.
- Multinational Team: The twelve monsters all come from different countries around the world — one each from North America (Baffin Island in Canada), Africa (Botswana) and Australasia (New Zealand), two from South America (one each from Aruba and Argentina), three from Europe (one each from Ireland, Portugal and Transylvania), and four from Asia (one each from mainland China, Hong Kong, Burma/Myanmar and Nepal).
- My God, What Have I Done?: In book 2, Burly accidentally makes Chelsea sick when he takes her out of the water and she stays out too long. He's horrified when Rilla points this out to him.
- Mysterious Note:
- The card Rilla received announcing she'd gotten a free membership to the Monster of the Month Club. She finds out late in book 1 that it was a present from family friend Mr. Tamerow, who thought the monsters were just plush toys.
- Later on, Rilla sends her classmate Joshua Banks (whom she has a big crush on) an anonymous Valentine, saying "Let there be love". He's not as oblivious as she thinks though, figures out it's her and sends back a signed note saying "Let there be friends" in the same style.
- Neat Freak: Rilla's Aunt Poppy (and not just because she has to keep things clean for their guests), to the point where when Rilla claims she's straightening things up in her room, Poppy views it as an acceptable excuse for running late for class.
- No Name Given: Sparrow firmly believes in this for animals, claiming it "limits them from all nature intends them to be". Rilla disagrees, and privately names the cats in the barn after sweet food and drink as a way of rebellion. Later, when Sparrow finds out Rilla had named the dog Taco, she reluctantly accepts it.
- Our Mermaids Are Different: Chelsea, the April Selection, is based on a mermaid, and spends most of her time in Rilla's tub.
- The Owl-Knowing One: Owl, the September Selection, is based on an owl and is very smart, getting smarter as he eats paper with new knowledge on it. Unfortunately, his ability to process new information is reduced when his glasses are removed.
- Parental Substitute: Mr. Tamerow, a regular guest at the Harmony House B&B, is the closest Rilla has to a father for most of her life.
- Perpetual Frowner: Icicle, the January Selection, who's pretty grouchy most of the time. Whenever he's interacting with Rilla, he almost always has a scowl on his face, though he tends to lighten up when he's caught up in a good book (only to switch back to looking grumpy if he's interrupted) or relaxing in the bathtub. He also tries to smile when he first sees Sweetie Pie and develops an instant crush on her, but this doesn't last long.
- Picky Eater: The monsters all have their own unique appetites. According to their individual cards:
- Icicle likes popsicles, frozen yogurt and iced lemonade.
- Sweetie Pie likes pink bubblegum, pink flowers (and as Rilla finds out, they have to be fresh, since she won't touch one that looks as if it's even thinking of wilting — she also spits out the stems, since they're green and not pink) and pink fruit punch.
- Shamrock likes clover, raw potatoes and green ginger beer. Rilla notes that feeding this one would have been tough, since clover is seasonal and she's not sure if the local convenience mart even stocks his drink preference.
- Chelsea (a mermaid-like monster) likes kelp, salt water and tuna. Luckily, feeding her isn't an issue, since canned tuna is easily found, salt water can be mixed up, and Sparrow keeps kelp supplements on hand.
- Burly likes baseball cards (snacking on Rilla's old card collection with her permission), cracker jacks and golf tees.
- Summer (a bird-like monster) likes sunflower seeds, cola and worms. Rilla doesn't have a problem getting the sunflower seeds, but is grossed out at having to dig the worms out of their yard for her (until she realizes Josh would gladly do it for her).
- Sparkler likes hot dogs, chips and apple pie. Josh, luckily, is able to provide his food, since these things aren't kept on hand at Harmony House.
- Butterscotch likes honey, granola and tofu, all of which are easily kept on hand at Harmony House, as Rilla notes.
- Owl likes poetry, numbers and literature. Rilla soon finds out this means he eats paper with these things printed on it, including her school books.
- Goblin likes candy corn, hot cocoa and spiders. As with Summer and worms, Rilla doesn't like the thought of having to get spiders for her.
- Cranberry likes acorns, pumpkin seeds and apple cider.
- Bow likes eggnog, pine needles and ribbons.
- Pink Means Feminine: Played to extremes with Sweetie Pie, the pink-furred February Selection, who loves everything pink (including her food).
- Planet of Steves: Played with — Rilla and her mother and aunt have unique first names. It's the middle name, Harmony, that's the same for all three.
- The Prankster: Owl pranks Rilla a few times, such as playing dead when she first meets him, and later after Sparrow and Poppy meet him for the first time (Rilla asked him to pretend to be stuffed while they're there, and after they leave, he pretends that he'd really gone back to being just a stuffed animal). He even gets Sparkler and Butterscotch to join in on it the second time.
- Put on a Bus:
- Rilla's classmate Kelly is absent from books 3 and 4, having been sent to private school instead.
- Another classmate, Andrew, would qualify for this trope if his departure didn't take place in the last book — during their last lesson before winter break, he announces that his family's going back to Australia for good during the break.
- Reasonable Authority Figure: Sparrow may be a little obsessive about her lifestyle, but she has her moments, such as relenting and permitting Rilla to keep the kittens in the barn after wanting to send them to an animal shelter, or keep the dog Taco after she finds out Rilla already paid his vet bills. She also wants to get rid of their internet service because of the price when it gets over-used (mainly because of Owl e-mailing anyone with "monster" in their screenname), but when she finds out Rilla was using it to find her long-lost father and had just successfully gotten in touch with him, she agrees to keep it and switch to a different billing plan to cover more usage instead.
- Rejection Affection: Icicle has a massive crush on Sweetie Pie from the moment he first lays eyes on her... and won't take the hint even after she decks him one.
- Roommate Drama: Some of the monsters who are active at the same time don't get along, such as Icicle and Sweetie Pie, Chelsea and Burly (though they get along better than the former two), and Sparkler and Butterscotch (he loves loud noises, she prefers peace and quiet).
- Secret-Keeper: Rilla herself for the fact that the monsters are alive. Rilla's friend Josh also finds out in book 2 when he comes into her room and meets Burly.
- Seers: Mother Lapis Lazuli, a mentor of Sparrow, has the ability to predict the future and read auras, making Rilla very nervous when she's around, since she's afraid the other woman will figure out the truth about the monsters.
- Shrinking Violet: Butterscotch, the August Selection, prefers lots of peace and quiet, to the point where she hides under the bed when it gets too loud. This is a problem when considering the fact that one of her active roommates is Sparkler, the July Selection, who loves loud noises.
- Sliding Scale of Living Toys: The monsters are supposedly just stuffed animals, but thanks to stellar alignments, they come to life at times. Icicle, Sweetie Pie and Owl are the only ones (as of book 3) to be alive when they arrived, the others were stuffed toys at first and came to life later.
- Smart People Wear Glasses: Owl, the September Selection, is easily the smartest of the monsters (and the only one to be able to speak English), and the only one to wear normal glasses.
- Sticky Fingers: Sweetie Pie keeps snagging things that match her likes from Rilla and, in one incident, another guest's room, forcing Rilla to quietly return them.
- Take That!: In-Universe, Aunt Poppy did this when she named the family goat Nancy after one of her ex-husbands' new wives.
- Tell Me About My Father: Rilla finally gets up the courage to ask Sparrow about her father in book 2 during her Calling the Old Man Out moment. Sparrow is startled, but admits she never realized how much Rilla thought about her father.
- Total Eclipse of the Plot: In book 3, it's mentioned that a solar eclipse will be happening on December 24 of that year. When it actually happens in book 4, the eclipse marks the only time in the series when all twelve monsters are alive at the same time.
- Wardrobe Wound: Downplayed at one point in book 1, when Rilla discovers one of her shirts has been torn in half, but is more bemused when she figures out why. It turns out Icicle and Sweetie Pie had both wanted it because it matched their interests — it was pink (Sweetie Pie's favorite color), and decorated with tiny popsicles (one of Icicle's favorite foods).
- When the Planets Align:
- The monsters are only active when stars line up in angled shapes like lightning, and only some of them each time — the April and May monsters are both from South America, so the same stellar alignment awakens them, while the June Selection (who's from Portugal) only awakens after Chelsea and Burly go back to sleep. Likewise, the July, August and September monsters are all from Asia and so are logically active together. Rilla does note in book 2 that she's not sure why Icicle and Sweetie Pie were active at the same time, since Botswana and New Zealand are far away from one another and couldn't be affected by the same star pattern.
- Another cosmic alignment in December, in which there's both an eclipse and a comet in the sky at the same time, allows all twelve to be active at once for the duration of the eclipse. The comet's presence also causes the October, November and December monsters to randomly activate and deactivate during the month.
- Who Names Their Kid "Dude"?: In book 1, Tina Welter mocks Rilla for naming a kitten "Pepsi". Then again, she mocks Rilla over anything she or her family does.
