The man in full, but less of his place and time

Henry IV poses a challenge for the modern biographer. His life and reign are well documented, yet the king himself remains an enigma. From none of the many sources do we get a clear picture of his personality. Shakespeare's poetic treatment merely adds to the mystery: despite writing two plays called Henry IV , he makes the king a central figure in neither. In the first it is Hotspur who dominates, and in the second Prince Hal. The king himself remains in the shadows - regal, but neither engaging nor charismatic.

Ian Mortimer meets the challenge of writing his life head-on. His book is a book with an agenda. Mortimer wants to prove the value and viability of medieval biography. Henry might be enigmatic, he says, but that is no reason to ignore him: he was the first monarch since the Conquest successfully to break into the line of legitimate royal succession; he is deserving of our sympathetic consideration. Mortimer's key assumption is that more than enough source material survives to allow us to construct a psychologically penetrating biography.

Mortimer's account of Henry IV's career is informed by a clear and compelling vision. He sees the king as able, intelligent and hard-working, always doing his best in difficult circumstances. Though he accepts that his seizure of the crown was not inevitable, he sees Henry and his cousin, whom he deposed, as born rivals. Richard's sidelining of the House of Lancaster, he argues, left Henry with little alternative but to act in self-defence. Once he had assumed the crown, however, his luck deserted him: he faced one problem after another; God appeared to be punishing him for his usurpation. Between 1403 and 1408, he fought off repeated and exhausting challenges from his rivals, the Percys. He spent his last eight years an invalid, with the political vultures gathering at his bedside and his son waiting to take his crown.

Mortimer's book is written with style and panache; he keeps the reader turning the pages. His vigorously paced narrative, however, comes at a price. He is apt to make rather uncritical use of his main sources, the chronicle narratives. Sometimes he treats the chroniclers' gossip as if it were hard fact. For example, he takes the reports of a plot on Richard's life on his return from Ireland in 1399 to mean that there actually was one; this may not have been the case. A more serious problem is posed by his tendency to draw on sources that suit his arguments while ignoring others that do not. In his account of Richard's deposition and its aftermath, for example, he makes heavy use of a French chronicle, the Traison et Mort , which has been shown to be largely invention. If the wealth of detail that Mortimer includes makes for a good read, it does not always make for good history.

Mortimer goes a long way to bridging the gap between popular and academic historical biography. He certainly succeeds in proving that medieval biography is possible. Yet the type of biographical writing he has developed here has flaws. His decision to concentrate on Henry the man leaves Henry the king acting in a social and political vacuum. It is all very well to highlight Henry's role as a decision-maker, a mover and shaker. The complex of influences within which Henry operated, however, goes almost unexplored.

What is needed here are thematic chapters exploring such issues as Henry's cultural and religious patronage, the role of his court and his use of the Lancastrian affinity - issues that cannot be treated in passing in a racy narrative. Mortimer has amply demonstrated his ambition as a historian. His book offers a wealth of challenging new insights into this fascinating but enigmatic ruler. It is not, however, a substitute for the full and balanced survey of the king and his reign that is so badly needed.

Nigel Saul is professor of medieval history, Royal Holloway, University of London.