Charlie starts high school after an extended stay in therapy because his best friend committed suicide the year before. His brother was the star quarterback, but is now in college on a sports scholarship. His older sister pretty much ignores him, and Charlie is more or less a social pariah, until he befriends the outgoing and charming Patrick, and his stepsister Sam, who are both seniors. They find Charlie's bluntness and awkwardness endearing, take him under their wing and introduce him to their clique of oddball friends.
Famously banned in several schools, and challenged frequently, probably because it shows a pretty honest take on high school life, with homosexuality, drunken parties, sexual experimentation, drug abuse, also alienation, depression, social anxiety, unrequited love and trying to find your place in the world - the book is structured as letters Charlie writes to an unknown recipient.
I must confess, I saw the film adaptation before I read the book, and therefore saw the characters pretty much as the are in the movie, but as the film is scripted and directed by Stephen Chbosky himself, that's not really a problem, because the book was adapted brilliantly. While I think I'm a bit older than the intended target audience, I would probably have loved it wholeheartedly if I discovered it as a teenager. Several of the teenage girls I teach are currently reading it because of the film, and they absolutely adore both interpretations of it.