The Troop

Read this in the recommendations of strangers on Facebook and it's one of the most gruesome horror I've read. The perfect diet book, actually, because you'll lose your appetite within the first chapter.
In a nutshell: Five boys and a Scoutmaster are on an island, soon to be joined by a man, ravaged by a mysterious disease, who's escaped from a military research centre. Things go bad, very bad, and it's pretty much up to the kids to survive.
The above does not do justice to the horror contained in the tale. Shades of Lord of The Flies with its frequent kids vs adults juxtaposition, but really, a solid, frightening Man vs Monster story.