Dead Heat - Patricia Briggs - Paperback

Alpha and Omega, book 4 out of 6, ongoing series

The Alpha and Omega novels transport readers into the realm of the werewolf, where Charles Cornick and Anna Latham embody opposite sides of the shifter personality. Now, a holiday to Arizona drops them into some bad supernatural business. 

Charles and Anna will discover that a dangerous Fae is on the loose, replacing human children with simulacrums. The Fae's cold war with humanity is about to heat up, and Charles and Anna are in the crossfire.