Rap, The L Sprague De Camps books are a Trilogy and are called

" The incomplete enchanter"set in Norse mythology, and Spensers "Faerie Queen"
"The Castle of Iron" set in The Castle of Otranto and ?Arabian nights?

" The Enchanter completed" set in Irish Mythology
The Co author? is Fletcher Pratt
and Powells books in Portland Oregon have what sounds like an omnibus edition containing all the "Harold Shea" books Listed Here