''And so the Cid rode out of the gates of history - and into legend.'' Charlton Heston''s epic film of 1961 captured some of the magic and romance of eleventh century Spain and added immeasurably to the legend. The bone-crunching jousts for the city of Calahorra; the blighted love for Jimena; the dead Cid still riding out against his enemies, the Moors of Africa - it all captivated a generation. But how much of it was true? Rodrigo Diaz from Bivar in Castile never lost a battle in the dozens that he fought. Some men branded him a traitor and a mercenary because he fought for Christian and Moslem alike, a self-serving warlord who fought for pay. Others saw him as a Christian knight, a crusader bent on winning Spain back from Islam. Dozens of legends have grown up around the man and later generations have hi-jacked him for their own purposes. He became a symbol of nationalism to General Franco''s Fascists during the Spanish Civil War in the 1930s, and there are statues to him as far away as New York. He was a king''s champion and lived at a time when men''s lives were nasty, brutish and short. ''But no one ever,'' as Heston''s film''s publicity said, ''was quite like him.'' In this gripping narrative, historian and crime writer M.J. Trow goes beyond the legends to the complex and fascinating realpolitik of the eleventh century world, as vibrant as our own is today.