In the late spring of 1910, a Special Branch officer in London tailed an unlikely visitor to a barber shop in the Caledonian Road. Days before, Kaiser Wilhelm II of Germany had attended the funeral of his uncle, Edward VII. The German emperor was not intent on having his hair cut. The barbershop was in fact a secret post office for an important German spy ring. Afterwards, British intelligence agents intercepted and opened its mail, enabling them to identify and monitor German spies across the UK. Four years later, at the outbreak of war, these enemy agents were rounded up, detained and German overseas intelligence hampered accordingly.

A peppering of vignettes of this sort holds the reader’s attention in Richard Aldrich and Rory Cormac’s overlong The Secret Royals. Piquant, poignant and unexpected details humanise the business of monarchy: the Crown Princess of Prussia lamenting the British ambassador’s inadequate knowledge of German politics in the 1870s on the grounds that he acquired “his information from bad sources, such as other silly diplomatists who understand nothing at all”; George VI’s enjoyment of the wartime MI5 reports produced by his favourite thriller writer, Dennis Wheatley; and the Queen’s present in 1974 to the football-mad 14-year-old Crown Prince Reza of Iran of tickets to a Rangers vs Hibernian match in Edinburgh as an alternative to hosting another visit from his father, whom she found “rather a bore” and “very heavy”.

The authors are academics specialising in international relations. Despite the obstacles to research in this area – archive material that is missing, classified or otherwise unavailable – their mastery of a subject that is extensive both chronologically and in its geographical scope is assured and impressive. The Secret Royals’ subtitle “Spying and the Crown, from Victoria to Diana” suggests its focus is Bond-style links between the royals and espionage, and the book contains both episodes and passages of writing reminiscent of colourful old-fashioned shilling shockers. It is not, however, consistently an account of monarchs and their spies: wittingly or not, it casts light on how British kings and queens have viewed their role, and the importance they have attached to their place in the nation’s foreign policy, once rooted in dynastic alliances and the densely interrelated fraternity of European royalty.

Queen Victoria and her successor Edward VII emerge as “meddling royals” as much as “secret royals”, with privileged access to alternative sources of intelligence through trusted family connections. In Victoria’s case, a degree of vehement intervention characterised much of her engagement with government. Her understanding of her constitutional position accommodated a degree of royal bossiness and foot-stamping that inevitably affected her attitude to Britain’s international relations and her personal security.

Once upon a time, thanks to its perceived underhandedness and element of deceit, spying was considered not quite British. The kinds of activity indulged in by intelligence officers, according to an early-Victorian newspaper editorial, “cannot be English, any more than masks, poisons, sword-sticks, secret signs and associations, and other such dark inventions”. But times were a-changing, and the creation of Special Branch in response to Fenian threats to Victoria’s life saw an increase in the number of government-funded intelligence officers in Britain from eight to 600 over the course of Victoria’s reign. Within a decade of her death, Charles Hardinge, head of the Foreign Office, had created a secret service bureau and the beginnings of modern British intelligence. Troubled international relations in the 20th century fuelled this demand and the requirement for increasingly sophisticated intelligence. Today, MI5 and MI6 play key roles in British national security; as if to underline the services’ relationship with the Crown, a senior member of Elizabeth II’s household is a former head of MI5.

For the generalist reader, The Secret Royals offers an intriguing alternative narrative of British royal history. The authors’ writing is simple and clear, but occasional banalities jar: the Prince of Wales, we read, “is particularly interested in action”, and elsewhere “The French Revolution burst on to the scene in 1789.” The book’s interest, however, lies in its subject matter and breadth rather than its style.

The Secret Royals is published by Atlantic at £25. To order your copy for £19.99 call 0844 871 1514 or visit the Telegraph Bookshop