The Faith trials. Vol. 1 : a novelization - James Laurence

Sometimes it's gotta be a drag to be the Chosen One. There are times when a girl would rather be at the mall, talking on the phone, or even doing homework than saving the world from unstoppable evil. Fortunately, Buffy Summers has always had a support system of friends, family and a father-figure Watcher to help her cope. But what if she hadn't?
She might have been just like Faith.
Faith, the 'shadow-self' Slayer, is strong, sexy and willful. When she first arrives in Sunnydale, Watcher-less and fancy-free, Buffy has doubts about tag-team patrol. As time goes on, though, she learns to find the fun in synchronised slayage, and appreciates Faith's zest for life... until Faith's impulsive approach to her calling takes her one step too far... and her inability to cope with the consequences eventually pushes her over the edge.