What Every Girl (Except Me) Knows by Nora Raleigh Baskin
Unlike most kids faced with the prospect of having a stepmother, Gabby Weiss isn't the slightest bit resistant to the idea. In fact, she wishes her father would hurry up and marry someone who knows about womanhood; someone who understands her worries about everything that is happening (or worse, not happening!) to her body. For a while, it seems like Cleo, her father's girlfriend, might fulfil this role. But when things fall apart, Gabby has to find her own solution. She decides to travel to the last place that she saw her mother, to try and reconstruct a memory of her. But what she finds there is something even better.