This book is pretty much perfect. I can't remember a time that I have been as emotionally devastated while reading the last third of a book. I actually had to take a break from it for a while because I was a sobbing mess. It's heartbreaking and you read it and wish that things that happen didn't have to happen, but this is the real world, and bad things happen like this every day. 
 The beauty of this book isn't even really in the strength of the main characters, although both are amazing (in particular Augustus who has catapulted himself near the top of my favourite character list) - the real strength in this book is the supporting characters. Characters like Hazel's and Augustus' parents, and Issac, and the characters within the book Hazel and Augustus love. Often secondary characters are weak and under-developed, but in this I felt like every character was given their moment of saying - this is who I am and I am more than a plot device. They felt like real people, which in a book like this, is exactly what they needed to be - because the tragedy of this book is that it is real and it affects more than just the protagonists. 
 It's just wonderful and I can't recommend it highly enough. I was a wreck afterwards, and thinking about small scenes still make me feel quite upset even now (view spoiler)[the scene where Gus goes to buy himself cigarettes and has to call Hazel for help still devastates me even a couple of days later. I don't know what it was about it - the fact that they had taken them off him, the fact that it was the point where he seemed to give up, or just the sheer agony of imagining while hoping desperately that you never have to experience that sheer feeling of hopelessness that he does. I don't know what it was about that scene, but I read it and had to put the book away for a day, just to try and steel myself for the ending. (hide spoiler)] It's heartbreaking, but it's perfect. Just, be prepared to shed tears...I find it hard to imagine that it is possible to read it without crying.