The tale of Charles I, more commonly known as Charlemagne, has been recounted by historians for centuries. Between 768 and 814, he ruled as king of the Franks, king of the Lombards and, most impressively, Holy Roman emperor. An enlightened reformer with a warrior-like ferocity, he united most of Western Europe and spearheaded the Carolingian Renaissance that enhanced arts and culture in medieval society.

It’s an incredible and almost unbelievable story. That’s why some historians now wonder if it really happened or if it’s a tall tale that would make Baron Munchausen laugh with sheer delight.

Janet L. Nelson, a professor emerita of medieval history at King’s College in London, is determined to resolve this issue in her intriguing new book, “King and Emperor: A New Life of Charlemagne.” With a small tip of the detective cap to Sherlock Holmes, she gathers the pertinent details (and there are many) in an attempt to unravel the mystery of who this king, emperor and man really was.

Nelson acknowledges that she’s sometimes “approached Charles from unfamiliar angles which can be unexpectedly illuminating.” She avoids the assumption that he perfectly fits within Thomas Carlyle’s “great man” theory and resists using Charlemagne’s moniker of “father” or “lighthouse” of Europe. Her book includes “an acceptance” that Charlemagne “colluded in the construction of his own story, thus making his biography in part an illusion.” She references German historian Johannes Fried, who has suggested that a biography “in the modern sense is impossible” because “not a single record of Charles’s words, or those of any of his five bed partners, or any of his children, has survived.”

If that’s true, his first biography, “Life of Charles,” published between 817 and 833 and written in Latin by a contemporary named Einhard, may not be the definitive volume of Charlemagne’s life we’ve been led to believe. Nelson’s “King and Emperor” could then serve an important modern role as an intellectual warehouse of ideas for his history — and leave existing historical interpretations open to debate.

His imperial coronation by Pope Leo III in 800 turned Charlemagne’s life from a story into folklore. The decision to crown him as Holy Roman emperor and bypass Eirene of Constantinople’s claim would seem controversial today. Yet as Nelson writes, “Contemporaries in East and West were willing to agree: feminine rule was a contradiction in terms.” The man who “already ruled an empire” had suddenly taken the mantle worn by the great Caesars.