This book was recommended to me as "Harry Potter for grownups," which turns out to be an accurate description. In it, Quentin, an overly intelligent guy who's constantly depressed because he's waiting for his real life to begin, for the big adventure that's going to make his life worthwhile, goes to a secret college for magicians.
From the outset, it seems that just the idea of going to a secret magic school and entering the hidden community of the magicians would make it like Harry Potter, and the inclusion of swearing, sex, and violence would make it for adults. But there's more to it than that. The way in which the magical world is presented echoes the prose in Harry Potter. Details of the magical world are presented simply as existing, then there is a line about how the students feel about it or the subtle effect it has on their lives, and then the detail is never mentioned again*. The difference is that in The Magicians, the students' response is some variant of "Ugh. Fuck that."
"Quentin spent very little time in the Brakebills library. Hardly anybody did if they could help it. Visiting scholars had been so aggressive over the centuries in casting locator spells to find the books they wanted, and spells of concealment to hide those same books from rival scholars, that the entire area was more or less opaque to magic, like a palimpsest that has been scribbled on over and over, past the point of legibility."The fact that little details like this are never mentioned again in a way makes the culture and traditions and history alive and immersive. The characters take them for granted almost as part of the scenery, which makes the world feel lived in.
*Of course, in the Harry Potter books no detail is safe from being a major plot point later. But in order for these details' importance to stay a surprise, they have to be buried in a bunch of other details that are unimportant or they'd be too obvious.
The Magicians extends this treatment of details to descriptions of characters. The descriptions are minimal and focus more on Quentin's response that the color of each character's eyes and hair and nose shape. This is something I love to read and love to write, even though I should do it more often than I do.
"Because he was plump and red-faced he looked like he should be jolly and easygoing, but in actuality he was turning out to be kind of a hard-ass."
The other thing I was really taken with was Lev Grossman's ability to tell the story of a series of books that the characters in The Magicians have read. The series is called "Fillory and Further" and are basically a fictionalized Chronicles of Narnia. A family of children visit the magical land of Fillory through a magic portal and there become kings and queens and talk to horses and rabbits before being send home by a pair of ram gods. The exposition of the plots of these books are interwoven into the plot of The Magicians with exceptional skill. Quentin loves these books, and so their plots and characters are presented in a similar way to the details I've already mentioned. Quentin knows these books backwards and forwards and doesn't need to stop and explain. He thinks about them often and he thinks about them well before they're needed in the plot of The Magicians. It's cool.
So the Magicians and Harry Potter present the magical world in similar ways, but The Magicians is for adults because of the themes it explores. Where Harry becomes an angsty teen, Quentin becomes a depressed man who has no one to understand his genius and his beautiful ennui (especially not women who could never understand him). Where in Harry Potter, his life changes to what it's meant to be when Harry goes to Hogwarts, Quentin is still waiting for something better when he goes to magic school, he's still waiting even after he's graduated. While in Harry Potter, bad people use bad magic and all the magic the good guys use is good and useful and harmless, The Magicians has a running theme that magic is dangerous and corrupting in how much power it gives to users. It lets them live lives of excessive luxury where they lose themselves. It lets them cause horrible accidents that get people killed and disrupt the boundaries between worlds.
"Just thinking about that place now gives me the howling fantods. They're just kids, Quentin! With all that power!...It's amazing that place is still standing."