Dead To The World by Charlaine Harris

Dead To The World is the fourth book in Charlaine Harris's series The Southern Vampire Mysteries, released in 2004. In Dead to the World, Sookie Stackhouse aids vampires Eric and Pam in their struggle against a coven of witches seeking to take over control of their area, and takes care of Eric after the witches erase his memory.
The novel Dead to the World opens on New Year's Eve, three weeks after the events of Club Dead. Sookie Stackhouse finds Eric running down the road close to her home, but he seems to have lost his memory. Sookie, initially reluctant to get involved in vampire matters once again, takes Eric in. Eric's second in command, Pam, is relieved at Sookie finding Eric, and explains a coven of villainous witches, some of them also werewolves, has arrived in Shreveport. Pam believes the witches to be responsible for Eric's erased memory. After Sookie's brother Jason bargains on a financial settlement, Sookie agrees to keep Eric in her house and care for him, as the witches are on the lookout for Eric and might harm him. The next day, Jason is missing. Sookie oversees the police investigation of her brother's disappereance, but personally fears the witch coven might've gotten hold of him.
Later on, Sookie informs werewolf Alcide of the witch coven being in town. Alcide fears that one of the pack members might have defected to the witches' side, but Sookie and Alcide then discover this particular woman's murdered body... (Summary by Wikipedia)