Shank, Michael H. 1987. "A Female University Student in Late Medieval Krakow" in Signs: A Journal of Women in Culture and Society: 12:373-380.
I thought I'd start off with this little gem because it encapsulates the sort of historic tidbit that could inspire an entire novel. There are no overt lesbian connections but the theme of cross-dressing to successfully play a male role in society is highly relevant.
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A 15th century account of a young woman attending Krakow university in male disguise. After successfully passing as a man for two years and nearly attaining her degree, she was unmasked due to the suspicions of a soldier who won a bet with his friends by forcibly undressing her and revealing her sex. She was taken before a judge but no one could find any complaint against her except the cross-dressing. After that she chose (from among unknown other options) to go into a convent where she became Abbess. A small amount of her earlier backstory is given: as a child she was a student along with some other children. When her parents died she came into an inheritance and that supported her decision to disguise herself “for the love of learning”. The source of the story is the autobiography of Martin of Leibitz (d. 1464) where he presents it as an event that occurred when he was living in Krakow.
Shank discusses the possible relationship of the themes of the story to hagiographical and moralistic literature popular at the time, as well as identifying details and connections within the story that argue for its historicity. He leans towards the latter.