Fintan's Tower — Catherine Fisher

Jamie was at the library looking for a book that was different, one he could get lost in. But he didn't mean it literally.

The Book with his own name in it leads Jamie and his sister Jenny into the Summer Country - a world of magic and danger, where even time behaves strangely; where Fintan's Tower has held its prisoner since the days of Camelot, and will keep Jamie and Jenny, too, unless Jamie can read the book right....