White Cat is the first book in a series called "The Curse Workers." In this series, Black is creating an alternate world, in which some people have special abilities that are exciting and terrifying to those who do not have such abilities. Thus, they are referred to as "Curse Workers."
Black has such a way with words. If you haven't read one of her books, then you may not realize this, and I suggest remedying the situation immediately. She has also written a trilogy of interrelated YA books: Tithe, Valiant, and Ironside. (Amazing.) She wrote The Spiderwick Chronicles, working with her friend and illustrator Tony DiTerlizzi - a fantastic series meant to be a good read, while still being more reader friendly, so that people aren't overwhelmed.
Black tends to write fantasy works, but she is able to make them believable. You read Tithe, and suddenly, the fairy world doesn't seem that far away. I read White Cat, and personally thought "Wouldn't it be cool if everyone wore gloves?" (See, you have no idea what I'm talking about - but you CAN know, if you purchase the book.)
The action moves smoothly, the characters aren't all perfect, aren't all stereotypes, and cause the reader to feel for them.
I cannot rave enough about this book. I highly suggest reading it, and I know I'm looking forward to the sequel Red Glove.