That depends on what you like. Here are some suggestions:
The books by Sharon K Penman set in mediaeval England and Wales. "Here Be Dragons" was the first one she wrote but you can read most of her books in any order.
Dorothy Dunnett has written several series set in mediaeval and Renaissance Europe.
Old fashioned gritty, George Shipway's "The Paladin"
Not my cup of tea but quite popular, Guy Gavriel Kay. He likes to set his stories in a fantasy world that's very close to historical setting. Most of his work is very mediaevally.
Verging on romance, "The King's Greatest Enemy" series by Anna Belfrage
Slow books that concentrate on the characters' inner lives:
"In a Dark Wood Wandering" by Hella S Haasse
"Kristin Lavransdatter" by Sigrid Undset
Since I can't tell your age, for YA readers:
Catherine Called, Birdie by Karen Cushman
The Door in the Wall by Marguerite de Angeli
"The King Arthur Trilogy: 'Sword And The Circle', 'Light Beyond The Forest', 'Road To Camlann' " by Rosemary Sutcliff (These are the straight up King Arthur stories set in a fictional Middle Ages, not her more well known, excellent Dark Age Arthur series which is a bit more grown up.)
"Proud Taste for Scarlet and Miniver" by E. L. Konigsburg