The Book of Night with Moon by Diane Duane

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Rhiow seems a perfectly ordinary New York City cat. Or so her humans think -- but she is much more than she appears. With her partners Saash and Urruah, she collaborates with human wizards to protect the earth from dark forces and maintain the network of magical gateways that connect to different realities. But amid this amazing secret animal world lies a danger that threatens not only the cats of the world, but humans as well.