Don't let the saccharine visuals (by Nate Taylor) fool you; this book is most definitely not for young children, unless you want to emotionally scar them for life. For adults, on the other hand, it's one of the best parodies of fairy tales in the vein (no pun intended) of the Brothers Grimm. As Rothfuss puts it, "I think of it as Calvin and Hobbes meets Coraline, with some Edward Gorey mixed in."
The book focuses on the titular duo, as well as a mysterious "thing under the bed". It has three separate endings; unlike the Choose Your Own Adventure series, however, it's meant to be read in a linear fashion. How the story ends depends on where you decide to stop reading, so you get a good ending, a bad ending, or..."the one with the teeth".
- Annoying Younger Sibling: The Princess does not like her new baby brother, Gubby, at all.
- Badass Adorable: In the sequel, the Princess effortlessly rips apart around fifty goblins for "hurting" Mr. Whiffle. Her baby brother also turns out to be one, at least to the extent that he can become a being of light and incinerate monsters even his sister is terrified of.
- Balloon Belly: The Princess gets one in the third ending, after eating the monster and kitten.
- Big Eater: In a rather dark manner. The Princess gorges herself on a whole monster roughly the size of an adult in addition to a kitten, looking quite satisfied afterward as she's picking her teeth and sporting a very full stomach.
- Big Sister Instinct: The Princess acts as caretaker to her baby brother, Gubby, in The Dark of Deep Below, and the conflict comes from him being abducted by goblins. Subverted in the first ending where the Princess leaves him to die. Played straight in the second and third endings where not only does she rescue him from goblins, but Gubby turns out to have little brother instinct and kills the monsters about to kill his sister.
- Bread, Eggs, Milk, Squick: The result of reading all the way through.
- Companion Cube: Mr. Whiffle to the Princess.
- Content Warnings: See Snicket Warning Label below.
- Crapsaccharine World: Oh, it looks nice, sure... until you get to the teeth.
- Cute Monster Girl: The Princess.
- Darkness = Death: Unseen monsters live in the Deep Below, and light is the only thing that drives them off. These monsters are so terrifying even the Princess is terrified of them.
- Derailed Fairy Tale: According to Patrick Rothfuss, this story was a real-life example.
- Even Evil Has Loved Ones: Mr. Whiffle to the Princess. Also in the sequel, her new baby brother, Gubby. Seemingly subverted in the first ending where she decides to leave him to be eaten by goblins and goes back to playing with Mr. Whiffles. Played straight when she realizes even if she doesn't like her brother, she can't bring herself to let him die, so she risks her life to save him.
- Even Evil Has Standards: The Princess will eat kittens and friendly, sapient monsters alive, and is shown in the sequel to make a game out of torture. However, in The Dark of Deep Below she's unable to abandon her baby brother to be eaten by goblins, no matter how much she dislikes him, because, "Some things, simply are not done."
- Foreshadowing: For the third ending in The Thing Under the Bed, here and there. For example, look at the castle gates when the Princess and Mr. Whiffle search for the kitten. The lock is on the outside.
- Fractured Fairy Tale: And how.
- Girls Love Stuffed Animals: Played straight and subverted; in the first part, the Princess stages a massive battle with her and Mr. Whiffle against all the rest of her dolls, which are vanquished. Mr. Whiffle, on the other hand, is loved and cared for by the Princess.
- Gory Discretion Shot: Subverted depending on whether you pass the second ending.
- Marzipan House: Where the Princess lives.
- Multiple Endings: As stated above, the books are meant to be read in a linear fashion, but will be different depending on which of the three endings you choose.
- The Thing Beneath the Bed
- Good end: The thing under the bed reaches for the Princess... and tickles her.
- Bad end: The Princess gets a kitten, but loses it. That night, she looks up to see something in the thing's hand, dripping down onto her face.
- "The one with the teeth": The monster was holding a melting piece of marzipan, and was taking care of the kitten in hopes of becoming friends with the Princess. She promptly devours both of them and makes their bones into a tent whereupon she has a tea party with Mr. Whiffle.
- The Dark of Deep Below
- First Ending: The Princess and Mr. Whiffle abandon her baby brother, Gubby, to the goblins and live happily ever after.
- Second Ending: The Princess feels guilt over abandoning Gubby and goes to rescue him from the goblins. While she kills the goblins, she, Gubby and Mr. Whiffle are trapped in the Deep Below in darkness where there are far more deadly monsters that can only be driven off by light. The Princess only has one match left and nothing for kindling... except for either Gubby or Mr. Whiffles.
- Third Ending: Gubby realizes his sister is scared of the monsters and reveals unknown powers, his entire body lighting up, incinerating the monsters and giving the Princess enough illumination to lead them out of Deep Below. The three return home and go to bed.
- The Thing Beneath the Bed
- Our Monsters Are Different: The "thing under the bed" sounds like a traditional boogeyman; it lives under the bed of a child and stays out of the light. The first ending subverts him when he simply tickles the Princess. The second ending plays him straighter after he seems to have gruesomely killed the kitten. The third ending turns it completely around, having the thing be friendly, while the Princess is the real monster.
- Parental Neglect: The Princess's parents are never seen in the first book, where it's said she lives alone in the Marzipan Castle. The sequel reveals her parents are around as she has a new baby brother but they're still never seen. At most the readers see the Princess's mother has a garden with a few tentacles and monsters tucked away and her father has a study locked behind a very ominous, rune-covered door.
- Precision S Strike: See Snicket Warning Label below.
- Schmuck Bait: This book looks very much like a children's story, from the title to the style of writing to the very cute illustrations.
- Snicket Warning Label: Some copies of the book bear a sticker with Mr. Whiffle's face and the text, "This shit is not for kids. Seriously." It's the only place where any profanity is used, but it's enough to get the point across.
- Sophisticated as Hell: See Snicket Warning Label above.