The book was very well written, pulled you into the story, and - at least I think- had an amazing ending. Now, onto what to look out for!
Like a lot of series, the last book had the most inappropriate content. Day (one of the main characters) is dying. At the beginning of the book he makes some questionable choices. He goes to a club and starts dancing with a random girl. Later they kiss and he notes that she tastes like vodka. Day also takes prescription cigarettes. Day and June kiss, then sleep together (though nothing is mentioned beyond starting to undress). A man kisses another man in an attempt to murder him. "D-mn" is used several times, along with "hell". "Bulls---" is used twice. The book has a fair amount of violence in it. Not enough that it bothered me, but you should just be aware of that.
I personally liked this book a lot more than I expected to and thought it was a great ending to the series, but I wouldn't suggest it for younger children.