Pagan's Crusade, Pagan's Vows, Pagan's Scribe and Pagan in Exile by Catherine Jinks, its' about the adventures of a young medival clergyman. Also Babylonne, companion to the Pagan books by Catherine Jinks
Catherine, Called Birdy and The Midwife’s Apprentice by Karen Cushman and Alchemy and Meggy Swan
Good Masters, Sweet Ladies by Laura Amy Schlitz
books about castles (our library has several titled Castles, including one with awesome cross-sections) and knights will likely be high-interest.
The Lost Years of Merlin by T.A. Barron
The Sword of the Rightful King by Jane Yolen
Catherine Called Birdy by Catherine Cushman
The Squire's Tale by Gerald Morris
I Rode a Horse of Milk White Jade by Diane Lee Wilson
A Single Shard by Linda Sue Park
Hunting of the Last Dragon Sherryl Jordan
Dragon Keeper by Carole Wilkinson
The Kite Rider by Geraldine McCaughrean