The Cats of Seroster — Robert Westall

Cam is quick-witted and clever - sometimes too clever for his own good. When he finds himself in a spot of trouble, he's keen to take a job that will get him out of the way for a while, even if it means he doesn't look at it too closely.

Tasked to take a letter to a far-off city - simple enough, you would think - he instead finds himself caught up in intrigue and dangers beyond those he was escaping, and in possession of a cursed knife he cannot get rid of. And that's before he even looks at the city's cats.

The city's very unusual, unexpectedly intelligent cats.

The city's unusual, intelligent and slightly magical cats, whose fates are now entwined with his.