Biography is a challenging form for a medievalist. The wealth of sources that modern historians take for granted in shaping portraits of their protagonists as private individuals as well as public figures - letters, diaries, newspapers, photographs - simply don't exist for the Middle Ages. Even the bare details of births, marriages and deaths can be controversial enough to provoke learned and sometimes inconclusive debate.
Yet the Yale English Monarchs series has produced several distinguished medieval volumes, among which Edmund King's study of King Stephen can now be counted. It is, he says, "a biography of King Stephen not a 'life and times'". This is true enough, if we understand that, medieval-style, it is the public story of a public life, a king's character and experiences deduced from his actions and reactions amid a complex political landscape.
And there was no more tortuously complex political landscape than the Anglo-Norman realm in the 1130s and 1140s. Henry I, the youngest son of the Conqueror, had hoped to leave his kingdom to his son William - but the young man drowned in 1120 when the White Ship struck a rock in Barfleur harbour. Henry was left with only a daughter, Matilda, as his successor, and did his utmost to bind his nobles with oaths to support her rights. But when Henry died, it was his nephew Stephen who seized the moment to stake his own claim to the throne.
Here, in 1135, Stephen comes into view clearly for the first time as he moves to the centre of the political stage: a man with an easy, generous temperament, and a leader with a striking ability to summon up irresistible energy and single-minded focus when an opportunity requiring quick action presented itself.
Stephen was the king's nephew, not his son, and was not even the oldest son in his own family, yet he secured the royal treasury and his own coronation before his cousin Matilda or his brother Theobald knew what was happening.
Becoming king, however, suited his talents more than the difficult business of remaining so.
King offers the telling vignette of Stephen delegating the task of addressing his troops at Lincoln in 1141 to one of his nobles, "since he himself lacked a commanding voice", the chronicler explains. "This may stand as an image of Stephen's kingship," King suggests. "He never made his voice heard." Certainly he never succeeded in stamping his authority conclusively across a country where his cousin Matilda always retained a foothold, in hearts and minds as well as castles and towns. Nor could he break her grip on Normandy, without which the future of the Anglo-Norman political realm could not be assured.
Tracing the path of Stephen's life through the years of civil war, "when Christ and his saints slept", as the Anglo-Saxon Chronicle has it, depends on the opaque evidence of charters and the persuasive yet problematic accounts of the chroniclers. King is an expert guide, with a forensic eye for detail and a sharp-witted way with an explanation for a 21st-century readership. ("The science of the exchequer," we learn, is "what a London taxi driver would now call 'the knowledge'.")
The only quibble I have with this fine book is that King's nuanced portraits of Stephen and his immediate family are not matched by his treatment of the king's rival Matilda. Instead, he follows the chroniclers in marginalising her, and criticising her conduct without any examination of the way in which her political actions were constrained by her sex - abandoning, as he does so, his astute observation elsewhere that a "good story" is not always the same as an explanation.
By Edmund King. Yale University Press, 352pp, £25.00 ISBN 9780300112238. Published 7 February 2011