The Eleventh Hour — Brian Stableford

Long ago, Iridia was isolated from the stream of time by magicians trying to preserve the golden age of magic forever. Unfortunately, the land's slow, comfortable decline is about to be forever interrupted by four teenagers, former playmates of Prince Edgar, whose paths are forever entertwined. Adam will become a soldier, and Francis will become a servant. But Sophie wants to be an archivist, a career not open to girls, and Michael is far too adventurous for a gardener. When Sophie borrows a magician's book, little does she realise it's the beginning of a great adventure that will change their lives--and world--forever!