Well it dosen’t help to say you can’t go wrong with any of them. Since you have The Winged Watchman I’ll suggest
Beyond the Desert Gate by Ray and others by her
Hittite Warrior by Williamson
God King by Williamson
Beowulf the Warrior by Serrillier
Son of Charlemagne by Willard
Beorn the Proud by Polland
Door to the North by Coatsworth and others by her
Madeleine Takes Command by Brill
The Happy Little Family series by Caudill
Books by Constance Savery
Year of the Black Pony by Morey
Books by Leonard Wibberly
Well…lots more, too, but that might get you started. ?