The Cats of Seroster by Robert Westall
The book is essentially a fantasy for older children. But a fun read non-the-less. The closest comparison I can think of right now is His Dark Materials. I think it is a small masterpiece of economical fantasy writing, with a great amount of plot, atmosphere, and world-building packed into a short book, which some of the writers of enormous fantasy series could learn from. And you know I am a sucker for cats!