"Cry Wolf" is the first book in the Alpha and Omega series by Patricia Briggs. It's a companion series to the Mercy Thompson books, set in the same world with some of the same characters, but taking things in a new direction! If you've read and fallen in love with the Mercy books, you'll love this series as well!
In "Cry Wolf" we meet Anna, and we get a chance to get to know Charles. Charles, as you may remember, is the Marrok's (the werewolf leader's) son and chief enforcer. In the Mercy books we've heard of him, but never really gotten to know him. Now, he's one of the main characters!
Charles goes into the Chicago pack and is forced to take out the leader, as well as several others, because they are completely out of control. They've been torturing members of the pack. Especially Anna. In the process the wolf in Charles and the wolf in Anna choose each other as mates. Normally the human's choose mates and hope their wolves agree, but this time it's happened backwards. This creates all sorts of interesting situations, especially because of how Anna was tortured, and because she is an Omega.
While trying to sort out their new relationship, Charles and Anna are sent out to track down a rogue werewolf. It turns out that not only is there a rogue on the loose, it's being controlled by a witch, a witch that has ties to a member of the Marrok's pack. It's going to take all the skill of Charles, Anna, and Bran to take down this witch!