From the back of the book:
Anna never knew werewolves existed, until the night she survived a violent attack...and became one herself. After three years at the bottom of the pack, she'd learned to keep her head down and never, ever trust dominant males. Then Charles Cormick, the enforcer - and son - of the leader of the North American werewolves, came into her life.
Charles insists that not only is Anna his mate, but she is also a rare and valued Omega wolf. And it is Anna's inner strength and calming presence that will prove invaluable as she and Charles go on the hunt in search of a rogue werewolf - a creature bound in magic so dark that it could threaten the entire pack.
This is the start of a spin-off series, Alpha and Omega, from the Mercy Thompson books, centered on the werewolves of the pack that Mercy grew up with. While Mercy Thompson, the viewpoint character of Moon Called, Blood Bound etc. doesn't turn up in this series, at least not yet, there are other characters which pass in and out of that series who are main characters in this one: Charles, of course, and his father Bran, for example. There's also Samuel, Charles' brother. At the same time, the main character, Anna has no part as of yet in the Mercy Thompson books.
Another signifier that this is a different series is the viewpoint the book is done from. To date, the entire Mercy Thompson series is written in first person from the viewpoint of Mercy herself. Cry Wolf changes through the different characters' perspectives, and is written in the normal form for a novel. I'll admit that I prefer this style, but I'm getting used to the other. It seems to be the normal one for Urban fantasy: Laurell K. Hamilton writes both her series in first person, and Kelley Armstrong's Bitten is as well.
Clearly, from the title of the series, the other books that are coming are going to be about Charles, the Alpha wolf, and Anna, the Omega wolf. I'm looking forward to it, as I've liked every book in the Mercy Thompson series, and I like the characters in this one to date.
Patricia Briggs has added some more neat twists to werewolf magic and behavior with this book, all leading neatly from the information we get in the Mercy Thompson series, and making sense as well. Not to mention a villain that you have to see to believe (and creepy to boot).
In terms of setting and time, Cry Wolf is set just near the ending of Moon Called, so for someone starting out with either series, you don't have to worry about being spoiled to later events, which is nice. It makes a fairly good introduction to the world the author has created, as well.
Overall, this is a good start to an interesting series, and I'm looking forward to the next book I've seen rumored on Amazon.com (though it's not showing up on the Chapters/Indigo site yet.
The other books in this series/world so far are:
Books and Other Thoughts: Alpha And Omega
Rhinona's Ramblings: Cry Wolf
Ceridwen's Book Lounge: Cry Wolf by Patricia Briggs