Babyface by Norma Fox Mazer (Good, 1991, Pbk, 165 pgs, Avon Flare)

At fifteen, Toni Chessmore was happy with her life. She had parents who adored her and each other. Her best friend lived next door and was everything a best friend should be. Toni's life was just about perfect...or so she thought. But that was before her best friend left for California, before they almost ended their perfect friendship over a boy, and before Toni heard a long-buried secret about her family that made her world come crashing down around her. Yet it was those bitter truths that let her see that nothing in life was perfect - including the people she loved the most.