Buffy the Vampire Slayer: Past Lives by Christopher Golden
A huntress is stalking demons in L.A. Normally this would be a good thing, except she's leaving survivors and telling them Angel sent her. Now Angel has an army of demons on his tall, hell-bent on revenge. What's a vampire to do? And how long can his human companions, Cordelia and Wesley, remain safe? Meanwhile, monsters are fleeting the carnage in L.A. and seeking refuge in Sunnydale, where they're being dealt with by the Slayer. Buffy, Giles, and the gang learn about the havoc, and Buffy determines that she help Angel in his hour of need; a decision that does not sit at all well with Buffy's vampire-hunting boyfriend, Riley.