BookSleuth®

I haven't read it, but maybe?  The Woman in the Wall by Patrice Kindl (1997).  "Exceedingly shy Anna, 14, narrates her life story. When she is seven, her mother tells her she must go to school. The school psychologist arrives at the run-down family mansion only to mistake Anna for a doll and somehow ends up with her in her purse. This is enough to impel the child to hide in a secret room she has readied overnight by putting up a false wall in the family library. Over the years, she adds new rooms, passages, a kitchen, peepholes; and no one notices. Although she continues repairing, baking, and sewing as her family requests, gradually her mother and older sister, Andrea, choose to forget her. When one of Andrea's ignored admirers sticks a love letter addressed to "A" into a crack in the stairs, Anna answers it, thus setting in motion a chain of events that lead to her discovery."

Amazon lists it for grades 5-8, so it's presumeably written for the 10-13 year old crowd.  As for why it's called Woman, School Library Journal notes "Kindl's messy ruminations on puberty" so the book's plot probably includes some reference to Anna's change from a child to a young woman?

I agree, it sounds very odd indeed.

I've read this one, too.

Here's my review from 2005:

The Woman in the Wall by Patrice Kindl: This is juvenile fiction about a little girl named Anna who is so painfully shy that she barricades herself within the walls of her family's home. She spends years there, until she discovers a love letter and must decide whether or not to come out of hiding. It's sort of a cross between children's and young adult fiction. My library system has it shelved in both areas. I'm not exactly sure how to describe this book. It's sort of magic realism. It's not quite fantasy, even though Anna is partially invisible when she's a young child and is small enough to accidentally be trapped inside a woman's purse. The book is an exploration of what it means to grow up and is definitely a coming-of-age novel. I'd certainly recommend it to children's literature fans.