Treasure of the Malkin - War of the Malkin series, #4

When demons rise, only a time-traveling feline can save the world. Henry is the Librarian, or was until a demon from the Horde thrust him into the future. Now he's a magical talking cat in a time where no such thing exists. After a being hit by a car, he strikes an uneasy friendship with an unassuming human named Liz, whose inquisitiveness rivals his own. Hoping to find a way back to his time, Henry asks Liz to grant him a final favor before he leaves: to visit the grand opening of the new Ancient Mysteries wing of the local library.
While admiring the exhibits from his own time, Henry discovers someone has plans to raise the demon Horde using an ancient magical artifact tucked away in the library's collection. With no one else to turn to, Henry and Liz work to stop the threat before the demons rise and bring the end of the world with them.