Whats the difference between the book and movie?

Some of the people who get killed and in what circumstances is different. The movie doesn't include a lot of scenes from the book and doesn't have much interaction besides the principal cast of John, Mr Crowley, and April. There still is interaction, just not as much as in the book. The ending is also different, but only slightly. Also, there is no inner monologue how there was in the book

Lot's of differences. Much more focus on John and his serial killer tendencies, what makes him tick, why he wants to kill, his rules to stay away from killing. Lots of focus on John and his therapist, also his therapist isn't some bird watching hipster (not to say he isn't a "cooler" type of character) but their meetings all take place in the therapists office. In the book John doesn't stalk Brooke like when he's watching her through their window, he purposely tries to stay away from her because she tickles his serial killer strings when she flirts with him. The biggest difference is the relationship between John and the Crowley's. The Crowley's view John as a son they never had, John does a lot of things for them, when he finds out that old man Crowley is the monster, he does some serious spying/following of Crowley's daily activities, much more than the film depicts. The violence is much more intense in the book, the demon has more kills and is more fleshed out as to why he has to kill to survive. There is a story-line with a hot National news reporter who comes on the scene after the second murder who starts digging around past news stories of unsolved murders and comes across a link with the black tar left behind, eventually Crowley kills him off in a different fashion so as to throw off his pattern of "taking" things from the body. This is an important subplot for the story as John is in the process of profiling the killer, ie: what is similar vs different, patterns etc, it throws John for a loop.

As you can see there are many differences, some subtle, some major, the film follows the very basic outline of the book which is why to me it felt like a very basic film. As a side note I just want to mention I happened to buy this book on a whim in a used bookstore about a month ago because I saw the movie trailer. I had no idea this was a young adult novel (it wasn't in the young adult section) and I did not get a young adult vibe from the novel AT ALL. If you are interested in the film and you like to read I highly recommend reading the book, it's much better than the film.