Immortal - Christopher Golden

When Buffy Summers encounters a vampire who is willing to sacrifice herself to let her companions escape, she doesn't have time to ponder the demon's uncharacteristic behaviour as she stakes it and watches its body explode to dust. Until the next night, when a new vampire confronts the Chosen One and her friends. A vampire who knows Buffy's name and can anticipate her style of attack. Who doesn't seem to fear the stake in Buffy's fist. A vampire whose name is Veronique.
But Buffy really knows she is in trouble when she discovers that Veronique is immortal. No matter how often Buffy might destroy her, it is only a matter of time before her ancient evil, wearing a new body, rises once again.