More a literary gumshoe detective than a conventional biographer, Tom Reiss has written the last of a trilogy of enthralling biographies. His first book was Führer Ex, co-written with a young East German, Ingo Hesselbach, who was involved in the neo-Nazi "scene" before repenting and shopping his erstwhile comrades. As a Jewish outsider, Reiss was well-placed to tell this story, as he was for his next book The Orientalist, the fascinating tale of Lev Nussimbaum, aka Kurban Said, a Jewish Azeri writer and wheeler-dealer who assumed the identity of a Muslim in Hitler's reich and Mussolini's Italy.

Now he tells a long-hidden story that sheds light on racism and the damage it does. And, although this time his research was conducted chiefly in rural France, it had its share of thrills. Reiss persuaded a French small-town official to blow open a municipal library's safe when the librarian  the only person knowing the combination  suddenly died.

The Black Count is a study of a hitherto obscure historical figure, General Alexandre (Alex) Dumas  not the famous Alexandre Dumas père, author of The Three Musketeers and The Count of Monte Cristo, nor the novelist's playwright son, who wrote the classic La dame aux camélias. Reiss's subject is the first Alexandre Dumas, who proves to have had a stranger life than many portrayed in his son's and grandson's fictions.

Dumas was born in 1762 in the French Caribbean colony of Saint-Domingue (modern Haiti). It was not an auspicious start. Dumas was a "batarde" - the product of a relationship between his aristocratic French father, Marquis Alexandre Davy de la Pailleterie, and a freed slave, Marie-Cesette Dumas. (Her surname, meaning "of the farm", was bestowed on her because managing a sugar plantation was her occupation as a freewoman.) Reiss is excellent on the colonial attitudes to race exemplified by Alex's father, who, after Marie-Cesette died when the boy was 12, though proud of his strapping son, sold him into slavery to pay for his passage back to Normandy. After six months of Alex's servitude, the marquis purchased his freedom and duly shipped him to France, to commence an education befitting a gentleman.

Alex proved adept at the skills expected of a young French aristocrat: especially fencing and horsemanship. His father remarried, however, and, possibly ashamed of his mulatto son, cut Alex off without a centime. Undaunted, the young man joined the army. During the French revolution he fought alongside other black men in a unit called the African Legion.

Dumas's military abilities were quickly recognised and he rose through the ranks from corporal to general in a little over two years, commanding a division of 53,000 soldiers when he was just 30. He fought in the many wars waged against revolutionary France and captured a key fortress in the Alps from the Austrians, who dubbed him Der schwarze Teufel (the "Black Devil") after he scaled an ice-covered rock wall in boots he had fitted with crampons. He was also noted for showing unusual moderation in the savage civil war against Catholic royalists in the Vendée. By the time Napoleon Bonaparte had emerged as the strong man from the chaos of revolution, Dumas was a dangerous rival.

Napoleon took Dumas on his disastrous Egyptian campaign as his cavalry commander, where the two generals fell out and Dumas quit his command. When he was shipwrecked off Italy en route home from Egypt, he was imprisoned by the Bourbon king, Ferdinand I of Naples and Sicily. Reiss suggests that it was deliberate foot-dragging by a spiteful and jealous Napoleon that delayed his rival's release. Ferdinand attempted to poison Dumas with arsenic and by the time he was finally freed, the man who could reputedly lift his own horse off the ground was a half-blind, half-deaf wreck. Dumas went back to the small town of Villers-Cotterêts where he had met his devoted wife Marie-Louise, and fathered the future novelist.

The France to which Dumas returned was utterly changed. Napoleon's military autocracy had extinguished the last sparks of revolutionary zeal. In the most poignant change of all Napoleon reintroduced slavery in Dumas's native Haiti, and crushed the republic of former slaves established by the Haitian hero Toussaint L'Ouverture. This even though Napoleon's first wife, Joséphine, was a West Indian Creole, from Martinique. (Earlier, Dumas had refused to command the expedition France sent against L'Ouverture, who ended his days in a Napoleonic jail.)

The story of Dumas's imprisonment and death in 1806 from stomach cancer exacerbated by arsenic (the same disease that would kill Napoleon) was told in The Count of Monte Cristo. Reiss has written a swashbuckling tale of his own.

Nigel Jones's Tower: An Epic History of the Tower of London is published by Hutchinson.