The Book of Night with Moon

Blurb

The Book of Night With Moon is a 1997 fantasy novel by Diane Duane. Although set in the Young Wizards universe, it was written as an adult novel. It centers on a team of cat-wizards.
This book takes place in between books 4 and 5 of the Young Wizards series. A significant portion of the book uses phrases or words in Ailurin, the language of cats, which contains 37 vowel sounds, far more than most or all human languages. These fragments of Ailurin are listed in a glossary at the back of the book.
The worldgating team at Grand Central Terminal is there to make sure that the four gates all behave correctly. However, the gates start malfunctioning and it is necessary for them to perform some larger interventions, even facing the Lone Power. They also have a new feline wizard on their hands with some hidden talents.