OK, I'll admit, the only reason I'd even heard of Becky Albertalli's Simon vs. The Homo Sapiens Agenda was due to the movie version coming out soon. (I was blessed with tickets to see a sneak preview a few weeks back. If you want to know, my review of that is here.) I'll do my best to keep comparisons of the two out of here until the end.
We are being narrated to by Simon Spier (rhymes with peer), who's a junior at Creekwood High School in suburban Atlanta. When we first meet Simon, he's explaining how it is he managed to get himself blackmailed by fellow student Martin about his apparent homosexuality. While Simon assures us the being gay part doesn't matter, he's concerned that should Martin release the screenshots of his e-mailed conversations with his pen pal Blue, Blue would be annoyed and quit talking to him. Martin wants Simon to hook him up with transfer student Abby, who recently moved to Creekside from Washington D.C. Abby has mostly integrated herself into Simon's AP nerd clique, although his good friend Leah isn't all that fond of her on occasion, likely due to Nick's infatuation with Abby.
We meet Blue through Simon's eyes, as someone who posted a not on the school's gossip site/Tumblr page, creeksecrets. indeed, conversations between Simon (who signs his as Jacques) and Blue show a friendship growing into a rather ephemeral romance/flirtation as they discuss music, Oreos, family issues, et cetera. Simon spends much of the novel trying to figure out who Blue is in real life, and indeed, one candidate does come out as bisexual later on, even if he isn't actually Blue.
We hear about AP English. We hear about the goings on behind the scenes of the school's production of Oliver. We get minor gossip about Simon's classmates, much of what isn't terribly unusual in my own experience. We meet his Freshman sister and his older sister who's in college.
Not long after Christmas, when Martin finally gets rejected by Abby, a post goes up on the Tumblr supposedly written by Simon that reads like a bathroom wall solicitation for gay sex. Which does set Simon up for a forced "Outing", as he more or less public acknowledges his homosexuality, something he'd really only told Abby about prior to it happening. Then begins the school harassment, as people do some not very nice things, as happens when you come out in high school. (I speak from experience. Although no one ever left dirty jock straps on my locker.)
Nick and Abby drag Simon downtown to a gay restaurant/bar, where an older man mistakes Simon for a college student, gets him drunk then sends him back to his friends when Simon lets it be known he's only 17. (This isn't a Winger song, thankfully.)
Drunk Simon, who found a present from Blue on his locker that morning, (Blue figured out who Simon was fairly quickly), wants to go home and get the shirt, having discovered what happens when you drink a lot after never drinking much previously. Drunk Simon then learns what angry parents are like when you come home drunk at 17. (Thankfully, having never drunk prior to turning 21, I can't really speak to that one.)
Anyway, after the last Matinee of Oliver, Simon send Blue an e-mail saying he'll be at the carnival in town sometime around 7 that evening. He wears the shirt Blue gave him, finding a note pinned into the hem, with Blue's number. Blue does at last reveal himself, riding the Tilt-A-Whirl with Simon. We get a few chapters after the reveal, showing a couple turning sickeningly infatuated with each other. (Seriously. I needed my insulin towards the end.)
A few things of note here: this story would again be different if Simon weren't from a fairly affluent family, if his family were say, Southern Baptists, if Simon belonged to another minority group.... I can't say how close to contemporary linguistics of modern teenagers (He never calls anyone ratchet or crunk, or whatever kids are saying these days) the narration is. And one of Simon's final thoughts rings true even as an adult. We're all houses with big rooms and tiny windows. On the other hand, when the bullying is going on, even as I was kind of reliving a few horror stories of my own, I wanted very badly to pass on MY words of wisdom. This too shall pass. It may pass like a kidney stone, but it will pass.
As compared to the movie, they tell similar stories, even if the sequences of events and the interpersonal relationships are different. Both ring fairly true on the important stuff though. And honestly, any resource that a young teen can get their hands on is a good thing in my book.