"THE PITTS," Nikolai Tolstoy observes, "were the wonder and glory of the 18th century." Two served Britain as prime ministers and Thomas Pitt, father of this book's subject, might have achieved the same eminence if invalidism hadn't plagued him. Raised to the peerage as compensation, he became the first Lord Camelford.

His son, Thomas Pitt, the second Lord Camelford, brought the family a different sort of celebrity: He ran away from school when he was 14. At 23, he commanded a Royal Navy ship, then killed a rival officer under dubious circumstances, and brought his notoriety to a still higher pitch by publicly caning a captain who had had him flogged during his first sea voyage. A brother-in-spirit to Byron's larger-than-life hero-villains, he died, at 29, as the result of a duel, with most of his contemporaries believing him a rogue without a single redeeming virtue, a bully, a killer, and a madman.

Tolstoy proposes to cut through and dissipate these prejudices, but the story would be worth telling even if it were detached from serious purposes and offered as a Graham Greeneish "entertainment." No Freudian, the author moves rapidly over the frustrations of Camelford's lonely childhood (his father's "concern was almost wholly with the duties of children to their parents") and the embittering neglect that sharpened his ambition to be famous and admired.

In a distinguished earlier book, The Secret Betrayal, Tolstoy investigated the duplicities of Stalin and his Western allies; now, writing of comparatively remote times, he is balked by repeated blanks in the record. The product, accordingly, is a scholarly biography that leaves tantalizing mysteries unresolved.

For example: Was Camelford working on a plot to assassinate Napoleon when he persuaded his colleagues in the House of Lords that he was a Frenchman in sentiment and an admirer of Danton, Marat, and company? Were his several visits to France the visible signs of murderous intent aimed at the lives of Paul Barras, a member of the Directory, and at "little Boney"? And was the pistol found on Camelford's person, a weapon capable of discharging nine bullets in 30 seconds, decisive proof that he was a would-be assassin? Rumor had it that Napoleon was alarmed by the Englishman's ability to outwit the spies set on his track by minister of police Fouche, but the fact that the French releaed Camelford whenever they trapped and jailed him suggests that their evidence was distinctly questionable.

Tolstoy describes Camelford's friendships, the loyalty of his servants, his kindnesses to the poor, and his fascination with manliness. "Poltroon" was his word for any Christian soul who turned the other cheek and, once he had hired a servant, he promptly tested the fellow's mettle by striking him. The servants who stood up to Camelford and pasted him were retained - their spirited fidelity was invaluable, and on one occasion they even saved his life by freeing him from a mob he was fighting singlehanded. His one fear was that he himself would ever be held up to ridicule as a poltroon. It became a deadly obsession and led to his fatal duel with the best shot in England.

The author seems to be one of those scholars who tie their most provocative pages to the adage, "Nothing ventured, nothing gained." His most interesting attempt is an effort to show that Lord Chiltern, the enigmatic firebrand of Anthony Trollope's Palliser saga, was inspired by Camelford. Tolstoy is convinced that Trollope "had access to original accounts of Lord Camelford," which were afloat in Victorian drawing rooms generations after his death. He also believes that the novelist supplied a vital clue to the real-life peer by suggesting "that Chiltern's aggressiveness and suspicion of others' goodwill stemmed from his upbringing in the house of a cold and distant father."

Had Tolstoy accepted that Camelford was an outright lunatic, the significance of The Half-Mad Lord would be distinctly limited. As it is, we find above and beyond his eccentricities a man who reflected the virtues and vices of his class and was in some ways a symbol of Britain. His taste for scientific experiment, his expert seamanship, his efforts to school himself in manliness by attending cock-fights and becoming a skilled amateur boxer, his ambition to excel by purchasing the best horses and wearing the best clothes - these traits, shared by many of his countrymen, help explain the nation's role in the Industrial Revolution, British successes against Napoleon, and the conversion of a small island into the fount of an unequaled empire.

The Half-Mad Lord should please readers who enjoy prose untouched by pedantry or flamboyance, a gift for analysis allied to a storytelling instinct, and the tact of a historian disinclined to parade his learning by way of a sprawling "life and times." CAPTION: Illustration, Engraving of the death of Lord Camelford (from "The Eccentric Mirror" reprinted in the book)