Lloyd Alexander and Diana Wynne Jones are my go-to-authors for this, have you read anything of them already?

King of Shadows by Susan Cooper (and also her more famous The Dark is Rising series).

Very out of print but worth it: Elizabeth, Elizabeth (UK title Robinsheugh) by Monica Furlong.

Not fantasy or historical, but The Mozart Season by Virginia Euwer Wolff.

You might like Weave a Circle Round by Kari Maaren, which is more recent than your cutoff but feels like an older book.