In a nutshell, it's about a girl who goes to prep school and is lonely. I've read books where the character has been more depressed than she is, but never where the isolation feels as familiar to me.
I recently put down Orhan Pamuk's Snow after fifty pages, because it felt so belabored; Prep in contrast didn't take cheap shots like forked tongues (I also just put down the Japanese "cult classic" Snakes and Earrings) or dead siblings (I'm due for my periodic Catcher in the Rye realization that it's not that good) or hospitilization of the narrator in a mental ward. It just feels right, with scenes that begin for believable reasons and end in believably unsatisfying ways. Though the book is basically a series of setups for subtle observations, the observations tend to come on the fly, and not in long explanations or descriptions (Philip Roth).
The author, a woman named Curtis (which prompted one Amazon reviewer who forgot to look at the back flap to write "the verbosity of his words really bored me to tears sometimes"), joins the tradition of Michael Drury and Wendell Steavenson, both women and respective authors of the great self-help book Advice to a Young Wife from an Old Mistress and the hit-and-miss book of adventures and profiles in Georgia and the Caucasus, Stories I Stole (statistically improbable phrases: "wait for the electricity", "thousand roses"). George Eliot doesn't count!