Ramon Menendez Pidal 
The Cid and His Spain [PDF ebook] 

This study of El Cid, first published in English in 1934, is by the leading authority on the medieval history and literature of Spain. The Cid occupies a unique position among national heroes. Others such as King Arthur and Roland are but shadowy figures in the historical record, but El Cid is very much better documented. This book also paints a striking picture of eleventh-century Spain, bringing out the importance of the country as a link between Christian and Muslim civilization.