I love Katherine by Anya Seton, about John of Gaunt's third wife, 1300s.

There have been a large number of mysteries set in medieval times. Some authors are with approximate years:
Maureen Ashe (1300s)
Simon Beaufort (Knight in Holy Land, 1100s)
Fiona Buckley (Eliz I)
Alys Clare (1200s)
Sara Conway (1200s)
Kathy Lynn Emerson (woman apothecary, 1500s)
Margaret Frazer (2 intersecting series, nun & actor, 1400)
Alan Gordon (Fool's guild, strange)
CL Grace (Dougherty pseud) 1400s)
Susanna Gregory (2 series, 1600s adnnd 1300s)
Paul Harding (1300s)
Karen Harper (Eliz I)
Michael Jecks (exTemplar, 1300s)
Bernard Knight (Crowner, 1100s)
Edward Marston (2 series: player, 1500s, also 1100s)
Pat McIntosh (Scotland, 1400s)
Candace Robb (Archer, 1300s)
Priscilla Royal (monk, 1200s)
Kate Sedley (Packman, 1400s)
Judith Tarr (fantasy with elves in 1100s, Rich I in some)