I just finished this one today, and it already holds a special place in my heart. It's one of the few books I would take as a physical copy with me to England (the others are Going Bovine and Beauty Queens by Libba Bray and the Princess Bride by William Goldman).
I think this is a book that everyone can connect with, no matter who you are. Stephen Chbosky wrote the screenplay and directed the movie adaptation, which is why the two versions are so similar to each other. It's like how William Goldman wrote both the book and the screenplay for the Princess Bride, but he changed the novel from being an unconventional fairy tale to a conventional one for the movie. Stephen Chbosky did no such thing, so the book and the movie are very similar in tone.
I saw the movie with my boyfriend, and he and I could both connect with Charlie. I connected with Charlie because I've also had instances where I've felt like I was on the outside looking in, and like nobody noticed me. Just the thought of other people having gone through the same things as I have comforts me in the fact that I'm not alone, no matter if I think I am.
The book is about Charlie, who is starting his freshman year of high school after having been held back a year for unspecified mental trauma. He is starting high school with no friends, his older brother off at his first year of college and his sister not really wanting anything to do with him at school. He eventually makes friends with a pair of seniors, Sam and Patrick. They start to include him in their circle of friends, making him feel like he belongs somewhere for the first time in a long time. With their help, he starts to participate in the world and in life instead of being on the outside, looking in.
If you wish to read this book, I have to warn you that it has almost every trigger warning under the sun: sexual assault, teenage pregnancy and abortion, domestic violence, questioning of sexuality, experimenting with drugs and alcohol, homosexual slurs, suicide, mental disorders, and probably some others which I've forgotten. I still encourage you to read the book because it's beautifully written, and touches on all the little things which happen while being a teenager.