If you read the previous two books in the series, just read this one too, you won't be disappointing. 
 While the previous books dealt with John's struggle and identity as a sociopath, here we see him in action accepting who he is and his mission in life. His personal life also develops entering a more serious relationship. Can someone without empathy be in a relationship? 
 Like the other books half the enjoyment is being inside this character's head, seeing how he sees and interprets 'normal' teenager and social situations. Anyone who struggled understanding people behaviours or emotions will identify. Book is full with humour, much of it dark. 
 The ending was great, you need to be a sociopath not to feel impacted by it. Good ending to a series but leaves room for a very different continuation.