Changes (Dresden Files)

“A can’t-miss entry in one of the best urban-fantasy series currently being published.”—Booklist (starred review)

As Chicago’s only professional wizard, Harry Dresden has faced demons, vampires, werewolves, dark sorcerers, and hosts of horrors from beyond the mortal realm. But nothing could have prepared him for this…
 
Long ago, Susan Rodriguez was Harry's Dresden’s lover—until she was attacked by his enemies, leaving her caught between humanity and the relentless bloodlust of the vampiric Red Court. She disappeared to South America, where she could fight both her savage gift and those who cursed her with it. Now, she needs Harry’s help more than ever.
 
For the vengeful Duchess of the Red Court has discovered a secret Susan has long kept from everyone—including Harry—and she plans to use it. To prevail, Harry may have to unleash the full fury of his untapped power—and he may have no choice but to embrace the darkness within himself.
 
Because this time, he’s fighting to save his child.