This is one of the very early Elizabeth Peters books, when Barbara Michaels was still finding her way with this new voice. It's not as polished as the later Peters books, or the Michaels books, but it's very enjoyable.
Laurie Carlson is in Chicago, laboring on her dissertation in mediaeval history, when a very alarming letter arrives from her great-aunt, Ida Morton. Great-aunt Lizzy, Ida's younger sister, has a new enthusiasm, fairies, and is acting even odder than usual. Can Laurie please come home? Ida, Lizzy, and their brother, Ned, effectively raised Laurie after her unreliable mother, Anna, lost interest in being a parent, and she can hardly say no.