During their extended honeymoon in Austria in 1937, the exiled Duke and Duchess of Windsor dropped in on Adolf Hitler at Berchtesgaden, his summer retreat in the Bavarian Alps. Each gave him the Nazi salute, while the führer gratified the duchess by addressing her as “your royal highness”, the title recently denied her by her brother-in-law, the reluctant new king, George VI.

“She’d have made a very good queen,” observed Hitler after meeting the homely woman who had precipitated the only abdication in the long and turbulent history of the British monarchy. “What a shame he is no longer king!” agreed Hitler’s sidekick Joseph Goebbels. “With him, an alliance would have been possible.”

Such is Andrew Morton’s account of a meeting of which Deborah Cadbury says there is no surviving record. Their publishers alone know why these two very different books on the same subject were published on the same day, offering insider tours of a shadowy wartime world that attempt to answer the hoary old question: was Britain’s ex-king a self-seeking traitor or a dopey dupe?

Cadbury’s root around the royal skeleton cupboard appears to have been prompted by the 2010 film The King’s Speech, to which there is a tacky reference on her dustjacket. Morton does not even mention the film, or the Australian speech therapist who helped the nervous new king overcome his stammer, Lionel Logue, to whom Cadbury devotes more than ample space.

So is the tabloid biographer of Princess Diana, Madonna, Tom Cruise, the Beckhams, Monica Lewinsky and suchlike taking an unexpectedly loftier line than the broadsheet historian of early fossil hunters, the Soviet-American “space race” and the “chocolate wars” between 19th-century Quaker capitalists? Cadbury is a former BBC producer, Morton a sometime red-top royal reporter. But his is the more compelling account of a murky episode about which neither the government nor the royals of the day wanted you to read.

Morton’s book seems to have been inspired by the “cover-up” of his subtitle (The Windsors, the Nazis and the Cover-Up), the “Marburg file” suppressed after the war, of which he gives a much more detailed and potent account. Because of its (baffling to its American allies) deference to its royal family, Britain’s establishment went to desperate lengths to avoid potential embarrassment for the monarchy.

What was the precise nature of the secret exchanges between the Nazi leadership and Britain’s ex-king concealed in this contentious file? Both books examine in tortuous detail, some of it hitherto unpublished, whether the Duke of Windsor really believed Hitler could reinstall him as a puppet king in a conquered Britain. Or was he simply intent, like many fellow appeasers, on avoiding another European war so soon after the last one against the same enemy, which had swiftly escalated into a calamitous global conflict vainly dubbed “the war to end all wars”?

If so, he was in good company. Meeting Hitler also impressed the British press barons Lords Rothermere and Beaverbrook, not to mention Britain’s first world war leader David Lloyd George, who called Hitler “the George Washington of Germany”.

Was Windsor plotting against his homeland? Under his formidable wife’s influence, he appears to have been more concerned to negotiate with the Germans the safety of their homes and possessions in occupied France, and of the absurd amounts of luggage required for them to roam the world in a style not enjoyed by their compatriots. Eventually, his fair-weather friend Winston Churchill conspired with his brother, George VI, to tuck the duke safely out of harm’s way as governor of the Bahamas.

It is salutary to be reminded that Wallis Simpson never wanted her “David” to abdicate. After a childhood miserable enough to render him a suicidal depressive, it was his own decision to ruin his life, wrongly assuming he could retain his plush royal lifestyle after disclaiming his birthright. Even so, he was banned from returning to England because George VI remained scared of being upstaged by his more confident, glamorous brother.

If Cadbury’s title is self-explanatory, Morton’s derives from the number of flowers sent daily to Mrs Simpson, mistress of the then Prince of Wales, by the prewar German ambassador to London, Joachim von Ribbentrop, supposedly marking the number of times they had made love. High British society, especially the Duchess of York who would soon become queen, regarded “that woman” as a gold-digger, blackmailer and Nazi sympathiser, perhaps even a spy.

In fact, Simpson seems to have been more of an avaricious harpy, never in love with the hapless man so inexplicably besotted with her. All four surviving Windsor brothers and their wives, meanwhile, not to mention their numerous siblings and cousins among the enemy, proved the very paradigm of a dysfunctional family.

Current concerns about an “interfering king”, as raised by Catherine Mayer’s scrupulous new study of Prince Charles, and the Guardian’s 10-year legal battle for publication of his “black spider” memos to ministers, ring through these narratives, notably in Edward VIII’s outburst during his brief reign in 1936: “Who is king here? Baldwin or I? I myself wish to talk to Hitler and will do so here or in Germany!”

Given the overwhelming logic for an elected head of state in this self-styled democracy, these revelations of the genetic flaws in the family still occupying that hereditary national role will do nothing to aid the royal cause. Quite the reverse.

Princes at War: The British Royal Family’s Private Battle in the Second World War is published by Bloomsbury, £25. To buy for £16 click here. 17 Carnations: The Windsors, the Nazis and the Cover-Up is published by Michael O’Mara, £20. To buy for £16 click here