Sherlock Holmes refuses to die. The "world's first consulting detective" lingers on like a restless ghost, nearly a century and a quarter after he first appeared in 1887.
His creator, Arthur Conan Doyle, first tried to kill him off in 1893. He supposedly fell to his death grappling his arch enemy Professor Moriarty at the Reichenbach Falls in Switzerland.
Public outcry -- and a shrinking bank balance -- persuaded Doyle to resurrect Holmes. Some 31 further Holmes stories were published, but by 1927, the great detective had retired to keep bees on the Sussex Downs.
Holmes, however, was destined to have an active afterlife. Countless movies, books, plays and even a smart-phone app have kept him alive and sleuthing.
Last year, there was also a TV adaptation starring Benedict Cumberbatch which placed Holmes in a modern, text-and-tweet setting.
And now it's been announced by the Conan Doyle estate that Holmes is to don the deerstalker once more: they have commissioned an "official" new Holmes mystery from Anthony Horowitz, author of the Alex Rider children's thrillers.
Horowitz's book will join a groaning library of sequels, pastiches, knock-offs and scholarly tomes, and will be filed alongside The Italian Secretary by Caleb Carr, a previous Holmes outing sanctioned by the Doyle estate in 2005.
The Holmes stories defy all publishing logic. There is no romance, definitely no sex -- many scholars think Holmes was a virgin -- and their hero is remote and aloof.
Worse, he is a depressive cocaine addict (taken in a 7% solution), given to scratching out melancholy solos on his violin. Yet his appeal endures. "Professional critics can't lay a glove on Conan Doyle," says thriller author John Le Carré, and he's right.
The stories conjure up a complete Victorian world, with its railway timetables, Hansom cabs, gentlemen's clubs and elaborate social codes.
The writing is plain and un-showy. It is "a perfect interplay between dialogue and description, perfect characterisation and perfect timing," says Le Carré.
Detail is tellingly used and tends to stick in the mind. We forget -- or never knew -- the addresses of Hercule Poirot, Miss Marple or Lord Peter Wimsey, but we remember that Holmes lived at 221b Baker Street.
We know he kept his tobacco in the toe of a Persian slipper, and that the initials "VR" (Victoria Regina) were spelled out in bullet holes on his wall.
We recall that he was suspicious of women -- "the fair sex is your department, Watson," he once said -- and was only ever defeated by one, Irene Adler, in A Scandal in Bohemia.
Sometimes, even his first name is enough to evoke the idea of brilliant deduction. "Ten out of ten, Sherlock," was the famous reply of a drunken journalist stopped by gardai and asked if he had been drinking.
Holmes himself is a brilliant creation. Watson tells us little about him. We know he has a brother (Mycroft), that he knows everything about some subjects (poisons, for instance, or tobacco ash) and nothing whatever about others (politics, or philosophy).
Throughout the four novels and 56 stories, Holmes remains elusive, either languishing in his rooms in despair that there is no criminal mind great enough to challenge him, or twitching and alive on the scent of a mystery.
If Holmes is the most memorable creation in detective fiction, then pairing him with the worthy, rather plodding Watson was inspired. Holmes, the nervous, highly strung thoroughbred, and Watson, the loyal, dogged draught horse, form one of literature's great double-acts.
Yes, the characters are brilliantly conceived, but the stories themselves are wonderfully paced and plotted. Many denouements have entered into popular culture: Holmes's solving of a case based on how far the parsley had sunk into the butter on a hot day (The Six Napoleons), or his deductions following from the fact that the dog did not bark in the nighttime (Silver Blaze).
The remarkable thing about the Holmes stories is that Conan Doyle did not set out to write them at all. He wanted to be a doctor. But, having put up his plaque in Southsea, he found that patients were slow in coming.
He whiled away the time toying around with a character based loosely on his old professor at Edinburgh University, Joseph Bell. Bell was able to deduce a patient's occupation, class and even drinking habits by observation alone.
Doyle -- whose mother, Mary Foley, was Irish -- found that writing paid better than doctoring. His first Holmes story, A Study in Scarlet, was published in Beeton's Christmas Annual of 1887, and was an instant hit.
Most of the subsequent stories were published in the Strand magazine. Many were illustrated by Sidney Paget. The imprinting of Holmes on the popular imagination owes much to the atmosphere and quality of Paget's drawings.
Conan Doyle took to writing historical fiction and science fiction, and began to tire of his "thinking machine". Holmes was, he said, like a foie gras of which he had once eaten too much, and ever after the very mention of the name made him sick.
Holmes's fans -- myself included -- would beg to differ. So would the thousands of readers who still write to the great detective at 221b Baker Street (now a hotel) asking him to undertake an urgent case.
And we shall look forward to the new adventure -- to be published by Orion in September -- and hope to be thrilled once more by the words: "Quick Watson! The game's afoot!"