Now You See Her by Whitney Otto

Kiki Shaw, a game show question writer, is about to turn forty. She doesn't mind that, except that she's also disappearing. Parts of her that were always there are vanishing, and no one seems to notice. As she contemplates this experience, Kiki makes certain discoveries about her life and those of the women closest to her. Perhaps they will all evanesce bit by bit, until they detect where they misplaced themselves and their once-promising lives

Goodreads: 3.15

Hardcover, 1994