Feline Wizard, book 1: The Book of Night with Moon
Diane Duane1997

Rhiow seems a typical New York City cat: pampered by her Upper East Side owners, permitted in good weather to lounge on the apartment balcony, never allowed to run free. Or so the humans think. Rhiow is much more than she appears. With her teammates Saash and Urruah, she works with human wizards, protecting the world from dark forces and helping to maintain the network of magical transport gates that connect all parts of the world. The Book of Night with Moon is the gateway to an amazing, secret animal world. In it we learn much about cats: they have a complex language, society, and history; they can call on skills unknown to their owners; they live lives of challenge and danger that culminate, if they are lucky, in a "tenth life" that equates to the human heaven. That tenth life is the fate of one of Rhiow's team as they, plus the foundling Arhu, find themselves caught up in a danger that threatens not only the cats of the world, but humans as well.