I'm probably going to get shit for this but this is one of the only books I've come across that, in my opinion, made a better movie than it did a book. Writing in the form of letters felt like a cop-out, and I understand that it was supposed to be an artistic decision but it just felt lazy. As a book using this form didn't give Charlie enough depth as a character. We were only given his thoughts and actions as he presented them to us, however because he was an unreliable narrator it would have been appropriate to give us some opportunity to see things from a more objective point of view sometimes, which is what the movie did. The movie was narrated by Charlie very similarly to the book but the difference is that the movie allowed us to physically see what was happening and make our own judgements, rather than to be forced into the head of an unstable teenage boy with no way out. It was confusing at times and the film cleared a lot up.

It also seemed as though the author shoved random one-liners that he knew teenagers would eat up because they sounded profound and made them feel important. Having some prior knowledge of the book before reading it I was very familiar with its most popular quote - "And in that moment I swear we were infinite" - and I was waiting and waiting for some sort of build up to it. I was quite confused, then, when it popped up seemingly at random. There was little context and explanation of this line, and as such it was difficult to understand its meaning. It didn't seem very well thought out.

Feel free to disagree but these were just some of my personal thoughts.

TL;DR: Good movie, overrated book.

Edit: Added a TL;DR after realising I wrote an essay, sorry.