Raven's Gate is the first book in The Gatekeepers series by Anthony Horowitz, who is perhaps better known for his Alex Rider series. Raven's Gate is a children's / YA fantasy book, although it borders on horror (toned down for a younger audience, but still horrifying enough that it does belong in the YA section).
Matt is a young delinquent who is caught up in an unplanned bit of violence and finds himself faced with a choice: go to Juvie or go to a experimental fostering program. He naturally chooses the foster home, although this turns out to be a mistake. His foster parent is a creepy old lady who makes him work all day and refuses to send him to school. Worse, she apparently has some sort of magical powers to keep him from escaping, and resorts to ever more drastic measures to keep him under control. At the same time, Matt is finding hints that he may have some sort of ability that he cannot control, and which does not seem to be doing him any good.
Over all, this is not one of the better books in the YA section. The writing style is somewhat off-putting, with short, telegraphic sentences, often stilted and choppy dialog, and an over-emphasis on all the 'dramatic' scenes. It seems to have been written in the theory that kids don't know good writing, but they surely do love angst and adventure, so hit those hard. Despite the 14-year-old protagonist and the high number of violent deaths, the writing style seems more appropriate to a fourth grader.
On the other hand, Horowitz is very good at building a creepy setting. He throws out eerie characters and events in handfuls, building a Charles Addams like village with a rich (although sadly neglected) cast of villainous characters. His bad guys are very well cast -- much more so than his good guys. I am not a big fan of scary stories, but there is a lot of very good spooky in this book.
Overall, between the good and the bad, this is the sort of book that I probably wont re-read and I don't know who I would ever recommend it to, but I'm also not sorry I read it. Given that it is an easy read and only 250 pages long, if you have a young teenager looking for a tale of dark magic and general creepiness, this book may well be worth a read... after they've read through the more obvious choices, of course.
The next book in the series is Evil Star.