Anyone who's read Rainbow Rowell's Fangirl will recognize Simon Snow. He's that universe's stand-in for Harry Potter, a magical Chosen One fighting an evil villain while attempting to attend a magic boarding school. Fangirl's protagonist Cath is obsessed with the series, and is the author of a very popular slash fic between Simon and Draco Malfoy stand-in, Baz.
So the mere existence of Carry On is a bit strange. Our universe already has a Harry Potter. We don't another (fictional) universe's Harry Potter too. It's too many levels of meta. An actual bestselling novel inspired by a fictional bestselling series that was based on an actual bestselling series. Add in the fact that Cath's story in Fangirl is called Carry On, Simon, and you've basically got an actual bestselling novel inspired by a fictional popular fanfic inspired by a fictional bestselling series that's closely based on an actual bestselling series.
My head hurts already.
And, to be honest, Carry On is let down by its close Harry Potter parallels. They're fun to read in small doses, until you think about how you're reading a published novel that's basically a rewrite of Harry Potter, and you wonder why you're not just rereading Harry Potter instead.
Hogwarts is now Watford. The Forbidden Forest is The Wavering Woods. Dumbledore is The Mage, and Hagrid is Ebb the Goatherd. Just as Harry was obsessed with Draco's secret activities in his fifth year, so Simon was obsessed with Baz's in his sixth. For the first two hundred or so pages, the parallels seem unending.
If you want to read another author's take on Harry Potter, and laugh at all the references, then the first couple of hundred pages of this book are for you. Otherwise, the first two hundred pages could have been cut down to about fifty pages. Nothing much really happened. The plot barely moves. It's just Harry Potter-ish things.
But once the plot does get going, Carry On gets good. Really good. The further we get into the novel, the less it clings to Harry Potter for support, letting its original material shine. Baz and Simon are great characters once they're let loose, as is Penelope, the Hermione stand-in, and Ebb. The plot is pretty twisty, the romance is compelling, and in the end, it's not very Harry Potter-ish at all... which is great, if you can get through the beginning without swapping it for Harry Potter instead.
There are a couple of places where the meta-ness works -- but it works in reference to Chosen One narratives in general, and not Harry Potter in particular. For example, let's talk Agatha, the Ginny stand-in. She's the Chosen One's girlfriend who's sick of being the Chosen One's girlfriend. She wants to be able to do normal things with her boyfriend, not have her life be in danger. She always ends up as the damsel, and Simon just isn't worth that amount of risk. She's sick of knowing that she's the girlfriend in Simon Snow's story, rather than being the center of her own, so she chooses to do something about it. She's the only character to step back and consider that actually, maybe, she's only 18 and doesn't need to be involved in saving the world.
But generally speaking, the Harry Potter aspect holds Carry On back, forcing it into a mold that we already know too well. It's like if, instead of The Force Awakens echoing A New Hope, with the lost hero on a desert planet who finds a droid with a mission, it was a new series called Supernova Conflict instead. It could work as a parody, a comedy film, but once it tries to take itself seriously, it can't work... and for all its jokes, Carry On wants to be a serious novel. It IS a serious novel, when it breaks its own ground. But the metaness of its setting holds it back.