Jan Eldredge - Nimbus

From the author of Evangeline of the Bayou comes a classic middle grade animal-fantasy adventure about a black cat who must go on a magical journey to be reunited with her beloved owner. For fans of Katherine Applegate, Rosanne Parry, and readers of the Familiars series.
Nimbus had a rough start as a kitten. But her fortunes changed the day she met Fletcher. The boy nursed her back to health from a life-threatening injury and adopted her.
But when Fletcher's aunt brings home a mysterious jar, an ancient nightmare demon is unwittingly unleashed. Despite battling the monster to save her friend, Fletcher's superstitious aunt blames Nim for the attack and secretly abandons her at a dump.
There, Nim is befriended by a theater-loving rat and taken in by a kindly witch named Agatha and her motley crew of cats. Nim soon discovers that her battle with the demon left her with special powers—but more than that, she begins to suspect that she has always been destined for something extraordinary.
To return to her beloved Fletcher, Nim will need to harness her magical powers and enlist the help of her new friends. Along the way, she will discover she may be braver and luckier than she ever imagined.