Where Wolves Talk - A Fantasy for Animal Lovers (Book One of Two) by D. L. Lewis
Kitten, an American tabby residing in England, is a frustrated cat. He knows his place in the world: he was born to kill. Killing, after all, is what felines are supposed to do. Confined within his Lady's house, however, the young fellow is deprived of the opportunity to hunt live prey. The mansion is a sterile playground for a predator; offering nothing more than furniture which allows itself to be brutalized far too easily. The ambitious cat is bored and hungry for a challenge.
Kitten learns of a passage hidden in his Lady's library: the Door, which leads to an unknown world. The cat has been told that the source of all evil dwells openly in this place. The feline is eager to fight the sinister personage and goes through the Door with no hesitation.
The tabby finds himself in what appears to be a forest like any other in England. It doesn't take long for him to learn that this is a very different place.
This novel can be read as a stand-alone book (doesn't end with a cliffhanger) but there is a second (concluding) volume available, entitled "Finale."