Arthur Conan Doyle became so irritated with Sherlock Holmes that he plunged his creation to his apparent death in the Reichenbach Falls. Public outcry and large sums of money combined to bring the great detective back to life (although even Doyle agreed that, while Holmes might not have been killed when he fell over the cliff, he was never the same man again).
The resurrection of Sherlock Holmes illustrates an occupational hazard of writing a series of crime stories. An author is likely to tire of his detective much sooner than his readers. The weight of their expectations becomes a straitjacket. The better the author, perhaps, the more he is liable to chafe against the restrictions.
Crime writers don't get much better than Michael Dibdin and Ian Rankin but, on the face of it, both are at risk of contracting the Reichenbach Syndrome. Medusa is Dibdin's ninth novel to feature Aurelio Zen, the shadowy investigator loosely attached to Italy's Interior Ministry. A Question of Blood is the 14th outing for Rankin's Detective Inspector John Rebus, the sardonic maverick of Lothian and Borders CID (not counting short stories and a novella).
Dibdin has come close to killing off Zen in the past, but in Medusa he's healthy and reasonably happy - living in Lucca with his pharmacist lover, Gemma. Cavers exploring a network of military tunnels in the Italian Alps stumble on the remains of a young man, seemingly the victim of an accident in the Seventies.
The consequences flood into the present, sending increasingly agitated ripples though the lives of four middle-aged people who once knew him: a reclusive antiquarian bookseller in Milan, an army officer's widow in Verona, a wealthy Venezuelan living in an Italian tax haven and a high-ranking officer in military intelligence in Rome.
When the Defence Ministry confiscates the corpse and imposes a news blackout, Zen is despatched to mount a low-key but rival investigation on behalf of the Interior Ministry. Soon he realises that the dark heart of the matter is concealed in "the secret network of events collectively dubbed the misteri d'Italia ... wormholes pervading the body politic". The narrative interweaves Zen's leisurely exploration of this particular wormhole with the frantic - and indeed fatal - attempts of the victim's friends to evade the nemesis that pursues them all.
Medusa deals with the pathos of old loves, redundant lusts and outdated beliefs. It is this that lingers in the mind rather than the plot, which is competent rather than compelling.
Dibdin hints brilliantly at the moral bankruptcy of the Berlusconi regime and all it stands for. His prose is characteristically supple, expressive and seductive. (Only occasionally does it trip over itself. What on earth is the meaning of "Nieddu took an almost physical distance by his look"?)
Unlike his Italian colleague, Inspector Rebus has never done anything in a leisurely manner, and his activities in A Question of Blood are no exception. The novel offers a jumbo-size helping of plot. An ex-SAS soldier guns down two youths in a posh school north of Edinburgh for no apparent reason, then shoots himself. Rebus plays an advisory role in the investigation, rashly (and inexplicably) concealing the fact that one of the victims is his cousin.
To make matters worse, much worse, he is himself under investigation because of the death of a small-time hoodlum in a suspicious house fire. Circumstantial evidence links Rebus to the time and place of the death. He's got the motive, too, and it doesn't help that his hands are badly burned.
Military intelligence is taking a strangely thorough interest in the school murders, and a self-seeking member of the Scottish Parliament is using the case to provide ammunition in his feud with the police. As the story progresses, Rankin adds dashes of drug-smuggling, currency-laundering, stolen diamonds, arms-trading and the seamy side of Anglo-Irish politics. Meanwhile Rebus's private life continues on familiar lines laid down in earlier books - heroic smoking and drinking, an ever-extending CD collection, deepening friendship with his friend and colleague, Detective Sergeant Siobhan Clarke.
A Question of Blood shows Rankin's gift for narrative. He writes with a natural rhythm which exerts an almost hypnotic effect, reminding us (as Tolkien once remarked) that the word "spell" originally means both a story told and a formula of power over minds. Some degree of enchantment is necessary because the plot depends on remarkable coincidences. But the novel undoubtedly radiates an alluring aura of authenticity: Rebus and his Edinburgh don't exist, but Rankin makes us feel they should. He is often very funny too, with an unforced and essentially masculine humour that springs from Rebus's awkward and combative relationship with the world around him.
Both books will please their authors' fans - and, between them, they hint at the principal perils lying in wait for the series detective. Dibdin, whose best novels show a remarkable inventiveness, has been content to cruise through Medusa. His strategy is to make his hero increasingly less visible in his own series. The focus of interest lies elsewhere. Zen doesn't saunter into the story until page 41, and even then is a curiously low-key presence. Does he belong here at all?
No one could accuse Rebus of being low-key. It's true that given his diet, his methods of policing and his relations with his superiors, his continuing survival in any capacity seems little short of miraculous. In a sense he has the opposite problem to Zen's: he's beginning to harden, to edge into caricature; at worst he verges dangerously close to self-parody. Perhaps Rebus and Zen - and their creators - would benefit from a change.
What should the future hold for these delectable but slightly weary detectives? Only a sadist would recommend a couple of one-way tickets to the Reichenbach Falls. But maybe it's time for Rebus to try something different - an extended holiday in Rome, for example - while Zen pays a reciprocal visit to Edinburgh.
Andrew Taylor's latest novel is 'The American Boy' (Flamingo)