What Every Girl (Except Me) Knows — Nora Raleigh Baskin

Twelve-year-old Gabby Weiss is in the market for a stepmother. If only her father would cooperate, Gabby would have someone to tell her what is and isn't happening to her body. For awhile her father's girlfriend, Cleo, forms a bond with Gabby. But when the adults break up, Gabby's hopes for a stepmother are shattered. Still, sharing feelings with a woman has awakened Gabby's curiosity about her own mother's mysterious death. Once and for all, Gabby is determined to discover the truth.