Animorphs: Despite initially trying to keep their relationship low-key, it was very apparent to everyone that Jake and Cassie were smitten with each other. Marco also sarcastically makes the same observation in an early novel regarding Rachel and Tobias.
Rachel:(threateningly) What did you say? Marco's thoughts:Like it was some big secret.
Taken to a comical level in L.M. Montgomery's Anne of Green Gables series. While Anne is completely oblivious to the fact that her former enemy and childhood friend Gilbert Blythe is madly in love with her, everyone in the town of Avonlea are perfectly well aware. After Gilbert proposes and she refuses, everyone reacts with disappointment. This happens so often for awhile, it leads to this conversation:
Anne: I wonder why everybody seems to think I ought to marry Gilbert Blythe. Miss Lavendar: Because you were made and meant for each other, Anne — that is why. You needn't toss that young head of yours. It's a fact.
Aunt Dimity: The consensus opinion of the villagers about Nell Harris and Kit Smith being a perfect couple is this trope, at least after the death of Prunella Hooper in Aunt Dimity: Detective. The process takes a few years (and a few novels), but after many postponements (for a variety of reasons), they wed at the end of Aunt Dimity Down Under.
In The Belgariad, everyone knows that Durnik the blacksmith is deeply and quietly in love with Polgara, an agelesssorceress and the World's Most Beautiful Woman. It's also made fairly clear in-text that she returns his feelings, though she isn't willing to admit it until he dies saving her life. Fortunately, not only does their love for each other save the universe by giving her the strength to hold fast against an evil God, Destiny really wants her to be happy, so he is brought back to life.
Captive Prince: Prince Laurent, who's well-known for being cold and guarded, is so much more at ease around Damen in the second book that his men assume they're sleeping together for weeks before their actual Relationship Upgrade.
The Chronicles of Prydain: The companions all seem to regard Taran and Eilonwy's marriage as an eventuality, even before Taran's willing to admit how he feels about her and while he still has no idea if she's remotely interested.
Cooking With Wild Game is set in a conservative, tightly-knit village, so when the local spinster invites an unmarried stranger to stay in her house for the foreseeable future, everyone knows why.
The evil overlord in Deltora Quest actually uses this to his advantage by using a fake long-lost sister to lure Jasmine to the Shadowlands, knowing the only thing that would convince Leif to cross the border was a quest to save his friends. Wisely, he used the friend with lady parts for added incentive. Apparently they were so obvious even the goddamn Shadow Lord could see it.
Fairly tragically, everyone knows that Molly is head-over-heels in love with Harry. For avarietyofreasons, this won't happen. After the events of Cold Days, though, Harry doesn't rule out the possibility due to the shifted power dynamics.
More conventionally, everyone from Harry's family to his perennial Friendly Enemy can see that he and Murphy have feelings for each other. This is taken Up to Eleven in Skin Game, where literally everyone other than Harry himself can see it, including Harry's own subconscious. More than one character refers to Murphy as Harry's girlfriend, to his consternation. Nicodemus even counts on this as part of his Batman Gambit to manipulate Murphy. They've even admitted to each other on occasion, but always convinced themselves that it would never work.
At the beginning of Earth Unaware Victor's family elders sent his second cousin Alejandra to another clan's fleet because they noticed that they were developing feelings for one another even though they hadn't realized it, and endogamy is forbidden. After it's pointed out to him Victor realizes it was true and spends much of the book mooning after her.
Very much the case in Greek Ninja, where everyone seems to notice the tension between Sasha and Daichi, whereas they are oblivious to it (or denying, like Sasha).
Griffin's Daughter: everyone knows Jelena and Ashinji are crazy about each other. Each denies it for differnt reasons: Jelena because she didn't want to get her hopes up that an elf prince could love a "hikui", (a half-elf), like her. Ashinji because he fears his sadistic brother will try hurt her just to spite him.
Ron and Hermione from Harry Potter. Harry starts seeing it around Book Four (as did readers, which is why the films start the Will They or Won't They? in Movie Two), and even basically shouts "Finally!!!" in his head after The Big Damn Kiss in the final book (followed immediately thereafter by "...but can we finish the bloody war first??").
Margaret de Noram: Don't you see it? It's plain as rocks to anyone else. You both sit in the same room trying so hard to ignore the other's presence that it's obvious you both want — no, need to talk to each other.
The relationship between Honor and Hamish Alexander. The first six chapters of War of Honor are full of secondary characters who are restraining themselves from telling the two principals that they're only fooling themselves.
And in the Jane Lindskold short story "Ruthless", this trope is exploited: The long list of people other than Crown Prince Michael and Judith who can see it include a pair of plotters who intend to use their feelings for each other against Queen Elizabeth.
The Hunger Games: It seems like everyone in Panem knows that Katniss is in love with Peeta Mellark before she does. Including the other guy in the love triangle.
In the Little House on the Prairie series, everyone but Laura herself seems to figure out rather early on just why Almanzo's willing to make the long, cold drive every weekend to bring her home from her teaching job. She's quite shocked when one of her friends clues her in, and initially panics. Fortunately, Almanzo sticks it out as a Dogged Nice Guy until she falls for him, too.
In Lockwood & Co., it is blatantly obvious to everybody who works for the titular agency (as well as a ghost in a jar functioning as a kind of Token Evil Teammate) that Lucy has a thing for Lockwood... except for Lucy and Lockwood.
In Loyal Enemies, even ghosts and villains can see it that Shelena and Veres are made for each other. Delirna's ghost flat-out asks Shelena how Veres is in bed (before either is even aware of any feelings), and the Big Bad tells Veres to get his monster girlfriend away from her.
The title character and Lord Sheftu in Eloise McGraw's Mara, Daughter of the Nile. Sheftu is extremely put out when he realizes this is the case.
You'd be hard pressed to name a character in Mo Dao Zu Shi who doesn't comment on Lan Wangji and Wei Wuxian's Unresolved Sexual Tension. The only reason it takes so long to be resolved is that Wei Wuxian is seemingly the only member of the cast who hasn't noticed Lan Wangji's obvious crush.
By the time you get to the last book in the Percy Jackson and the Olympians series, it's pretty obvious that everyone around them can see that Percy and Annabeth are in love with each other. It's made especially apparent towards the end of the book when they finally kiss and Clarisse growls, "Well, it's about time!"
It's actually a given that they're going to end up together since the goddess of love herself, Aphrodite, tells Percy she's going to set him up with Annabeth, but no one else heard it but him.
In Pride and Prejudice, meanwhile, it's played around with more. Everyone who sees Elizabeth and Mr. Darcy spend any time around one another over the course of most of the novel is convinced that they're practically a confirmed item (whether they like it or not). On the other hand, Elizabeth's immediate family are uniformly taken completely by surprise when Darcy proposes, and she has to do a lot of convincing before her older sister and her father will believe her when she says she really does want to marry the man.
In the Prince Roger series, Roger and Sergeant Despreaux spend a novel and a half in UST, partly because they're part of the same military unit and partly because she's one of his bodyguards. The other soldiers, instead of acting to stop this, set up a betting pool about when they'll get down to business.
Happens in The Princess Diaries. When Michael and Mia finally confess their feelings to one another, Lily comments that everyone already knew how they felt, and that they were only people who weren't aware of it.
Everyone opposed to Elinor's and Edward's budding romance in Sense and Sensibility is convinced that the two will marry, no matter how much the two try to hide it (particularly from each other).
Captain Vorpatril's Alliance centers on what the principals think is a quickie marriage of convinience between a runaway Jacksonian Mafia Princess and a well-connected Barrayaran Officer who considered it a better option than the former throwing herself off his balcony to escape Immigration. Neither his relations (including the guy he thought he had roped into signing off on the divorce) nor hers miss the feelings they have for each other.
In Shards of Honor, Cordelia and Aral can't take their eyes off each other, and she reflects they'll have to work on that. A doctor calls Aral out on the unspoken relationship.
In The Witchlands, Merik is the last person aboard his ship to realize he has feelings for Safi. It takes Kullen (who's bedridden for most of the journey and still sees what Merik doesn't) suggesting that the two of them have a "tumble in the bedsheets" while on land for Merik to realize that yes, he's been attracted to this woman for quite some time now.
Worm has Taylor heavily crushing on Brian, to the pint where Alec, an emotionally stunted sociopath and Rachel, whose brain has literally been rewired to canine psychology, think it's obvious. Brian remains oblivious to this as he's too focused on providing for his chronic runaway younger sister (who also noticed it immediately the first time she saw them together). In fairness, Taylor has spent the last two years being abused and emotionally neglected, so it would be strange if she didn't fall for the attractive beefcake who goes out of his way to make her feel welcome, and is one of the few people to treat her with warmth and respect.