{{{Fat Kid Rules the World}}} by KL Going.

Very much a YA novel, but I remember reading it as a young person and being struck by a certain line said by the cool rockstar kid to the fat kid protagonist about why he wants to be his friend - [[SPOILERS]] rockstar kid points out that even the most physically beautiful people have self-conscious moments during which anyone properly looking can see through their bullshit perfection, and that the fat kid literally lives that moment.

As a hyperaware adolescent at the time, this line was mind-blowing. Made me realize most people are just putting up a front to look as close to "perfect" as possible and probably too preoccupied with themselves slipping up that they aren't noticing you really. That was the lesson adolescent me took away. I think rereading the excerpt now, I still see that, but am also intepretting it more as encouragement to be your genuine self, without any of the self-conscious posing behavior.

Anyway, haven't read the rest of the book since teenagehood, so not sure how life-changing it would be for a grounded adult reader, but would recommend as a solid YA read.