The book, which enjoyed immense popularity in the years before World War I, is an early example of the espionage novel and was extremely influential in the genre of spy fiction.

"[1] All of the physical background is completely authentic – the various Frisian Islands and towns named in the book actually exist and the descriptions of them accurate (often, from the author's own experience).

Navigating a small boat under these conditions requires a specialized kind of skilled seamanship, of which the character Davies is an unsurpassed master, and the descriptions of his feats are of abiding interest to yachting enthusiasts, quite apart from their role in the book's espionage plot.

Having failed to interest anyone in the British government in the incident, Davies feels it is his patriotic duty to investigate further on his own – hence the invitation to Carruthers.

Taking advantage of a thick fog, Davies navigates them covertly through the complicated sandbanks in a small boat to investigate the Memmert site.

He overhears von Brüning and Dollmann discussing something more than treasure hunting, including cryptic references to "Chatham", "Seven" and "the tide serving", and hears of a rendezvous at the Frisian railway station, several days ahead.

But instead of embarking from Amsterdam to England, he doubles back, returns to Germany in time to be present at the conspirators' rendezvous (to which Dollmann, significantly, was not invited).

The only explanation is that they are preparing to secretly transport a powerful German army across the North Sea to invade Britain's east coast.

A postscript notes that the Royal Navy is finally taking countermeasures to intercept any German invasion fleet and urges haste.

"[2] It was a notable influence on John Buchan, and on Ken Follett, who described it as "an open-air adventure thriller about two young men who stumble upon a German armada preparing to invade England.

They’ve tinkered with fishermen, and merchant sailors, and yachting hands, but everyone of them ought to be got hold of; and the colonies, too.In fact the influence the book had on the formation of the Volunteer Reserve has been overstated.

The 1903 Naval Forces Bill authorising the formation of the RNVR was introduced to Parliament in March 1903 (the novel would debut in May) and was passed in June.

The formation of a naval reserve for civilians had been heavily discussed since the Boer War and its formation came after a concerted political and press campaign led by Thomas Brassey and Charles Chadwyck-Healey - in his book Childers was espousing the arguments made in favour of the Volunteer Reserve but the novel was by no means the source of the idea or the force behind its creation.

[9] Childers would in fact be commissioned into the RNVR on the outbreak of the First World War Similarly, the belief has grown that the book was responsible for the development of the naval base at Rosyth, but the chronology here is also incorrect; the purchase of the land for the Rosyth naval base was announced in Parliament on 5 March 1903, two months before the novel's publication, and some time after secret negotiations for the purchase had begun.

[14][15] The novel contains many realistic details based on Childers' own sailing trips along the East Frisia coast, and large parts of his logbook entries from an 1897 Baltic cruise "appear almost unedited in the book".

[16] In August 1910, inspired by the work, two British amateur yachtsmen, Captain Bernard Trench RM and Lieutenant Vivian Brandon RN, undertook a sailing holiday along the same section of the Frisian coast, during which they collected information about German naval installations.

[17] It is noteworthy that the demonization and vilification of Kaiser Wilhelm, which would become a staple of British propaganda in later years, are conspicuously absent from The Riddle of the Sands.

The Riddle of the Sands (1979) is a film adaptation of the book, starring Michael York as Charles Carruthers and Simon MacCorkindale as Arthur Davies.

[20] A 90-minute BBC Radio 4 adaptation was broadcast in January 1994, as part of the Saturday Night Theatre strand, starring Laurence Kennedy as Carruthers and Charles Simpson as Davies.