Any antidote to the potboiler novels set in 16th-century England and the risible TV dramas about the Tudors is welcome.

Enter, then, Catherine Fletcher, who has written a scholarly but wonderfully entertaining book. It proves beyond any doubt that the reality of Tudor history is far more interesting and exciting than the fictions. Into the bargain, Fletcher has introduced us to an unfairly neglected figure who, until now, has "rarely [escaped] the footnotes". The man in question is Gregorio Casali, who served as Henry VIII's representative in Rome during an unusually tense period.

The story of Henry's run-in with the papacy is well known. Catherine of Aragon had failed to provide Henry with a son, so the king sought out new marital pastures. Ordinarily, this would not have been a major obstacle, and the pope, Clement VII, would probably have succumbed to Henry's arguments about the illegality of his marriage to Catherine.

These were not ordinary times, however. Italy was in chaos, largely thanks to the Holy Roman Emperor Charles V, who just happened to be Catherine's nephew. The pope's room for manoeuvre was severely curtailed and a quick divorce was out of the question. And so the circus began. Diplomatic initiatives were launched, threats bounced back and forth, and by the early 1530s an infuriated Henry had broken with the Roman Church.

The role of Casali in all this was pivotal. English diplomats (lots of them) came and went between 1527 and 1533 but Casali was an almost constant presence, and he worked very hard to do Henry's bidding on the banks of the Tiber and across the Italian peninsula. There was the campaign to ensure the official investigation of Henry's actions took place on English shores (successful, for a while), the attempt to prevent proceedings moving back to Rome (a failure) and, latterly, a major effort to secure support from Italian academic theologians.

This might all sound very technical, perhaps even a little dull. It really isn't. It offers an opportunity to explore the strategies of early modern diplomacy and this makes for a murky, fascinating tale. There were attempts at bribery, forged letters and reports, intercepted mail and no end of professional rivalries. Casali was in the thick of things and Fletcher's portrait of him is superb. Many questions remain unanswered, but this is hardly Fletcher's fault: the sources are often scant. We'll never know if some of the charges levelled at Casali by his enemies – sabotage and overstepping the diplomatic mark, for instance – were justified, but thanks to Fletcher we can be certain of one thing: Casali was an intriguing and, by dint of historical circumstances, truly significant figure. He was no intellectual giant or diplomatic genius, but he was caught up in one of the era's most awkward moments. He warrants our attention.

In many ways, it is the ordinariness of the man that is most appealing. These days, the brightest and the best aspire to ambassadorial careers and, when successful, they acquire many perks and a healthy dose of prestige. In the 16th century, some of the same rules applied: smart people often got the jobs and they enjoyed more privileges than most. In other ways, the diplomatic scene was very different. In the vast majority of cases there wasn't much money to be made: the entertaining, gift-giving and travel tended to leave emissaries out of pocket. It was also a far more perilous and precarious profession. Casali therefore did very well to cling on to influence and make a crust for as long as he did.

Perhaps the greatest joy of this splendid book is that it dwells on context. You'll learn a great deal about why the squabbles between Charles V and the king of France made Italian and papal politics such a muddle. You'll emerge with a keener sense of why the dynastic priorities of Henry VIII ("a mid-ranking northern monarch, a player on the European stage but far from the star of the show") managed to cause such a fuss. With any luck you'll switch off your TV and rely instead on the hard work of experts who can write very well. How snooty of me! How little I care! When it comes to the Tudors and their accurate portrayal, I'm willing to be a little crotchety.

Our Man In Rome: Henry VIII And His Italian Ambassador

Catherine Fletcher

Bodley Head, £20