I don't remember the name or author, though I remember it was post-NFA. Xander finds Spike, maybe in LA, and they get together. I dont think Xander tells the Scoobies that Spike's around, but somehow Buffy finds out and comes to see him. Buffy tells Spike she actually does love him, but he tells her he and Xander are together. (In the scene I do remember, they are sitting in a diner, I think.) Then he tells her that Angel has Shanshu'd and to go find him.