Did anyone really believe Ian Rankin was going to stop writing about John Rebus, the cantankerous, alcoholic detective who was retired by his creator, to much mourning, in 2006? In retrospect, we should all have known better: Rankin was always going to find a way to keep Rebus on the page. He's just too good a character to let lie.

In Standing in Another Man's Grave  the book is dedicated to the late Scottish singer Jackie Leven; the title is Rebus's mishearing of Leven's line "another man's rain"  we find Rebus back on the case, working for the serious crime review unit, albeit in a civilian capacity. Still smoking, still drinking, he's looking into cold cases, working "with the long dead, murder victims forgotten by the world at large", when a woman arrives with a story. Her daughter vanished from Aviemore, on the A9, in 1999, and she believes the disappearance of a string of other young women from towns near the road over the next 12 years are linked. She's got nowhere with her theory but Rebus decides to listen, particularly as an ongoing missing person case also has links to the same road.

And what do you know? His old protégée, Siobhan Clarke, is on that job, and the pair embark on an investigation which will see Rebus travel the length of the A9, its "thousands upon thousands of inland acres of wilderness", in his battered old Saab, running into buried secrets and old nemeses, enraging his superiors and crossing line after line in his quest for justice.

"You were a bastard back then too. Just not so fat and old," he is told by one former convict he helped to put away. "Hard to disagree," he replies. Rebus hasn't changed; he's as sharp, petty, curmudgeonly and likable as ever, one minute headbutting gangsters in the groin, the next winding up his blinkered bosses. What has changed is the world he operates in. No longer a police officer, he has to stand aside as others interrogate and make the decisions. He's described as a dinosaur, and we see him bemusing younger police officers by writing out notes rather than emailing them.

"John Rebus should be extinct, Clarke. Somehow the Ice Age came and went and left him still swimming around while the rest of us evolved," says Malcolm Fox, a teetotal inspector who investigates wrongdoing within the police force, and who has his sights set firmly on Rebus. "I know a cop gone bad when I see one. Rebus has spent so many years crossing the line he's managed to rub it out altogether."

Fox is Rankin's new series character; he's already starred in two novels of his own, The Complaints, and The Impossible Dead. Rankin is clearly enjoying pitting his leads against each other here  in one scene, when Rebus instigates a first meeting with Fox after learning he's being investigated, the contrast of the pair, Fox with a glass of tap water and a banana, Rebus stifling belches of Irn Bru, is glorious. "Most cops looked like cops, but Fox could have been middle management in a plastics company or Inland Revenue," thinks Rebus. This very contrast might prove problematic for the author in future books; Fox was a compelling hero in previous novels but he's pallid in comparison with Rebus. Not to worry, though, because Rankin leaves the door wide open for more from his former DI, and thank goodness.

The 28th work of fiction from Scotland's most successful crime writer turns on five ominous disappearances  young women who went missing close to the A9 road over a 10-year period  and an entirely welcome reappearance. DI John Rebus, retired in 2006 after 18 cases in line with Ian Rankin's hyper-realistic policy of ageing his characters in real time, has now returned to work, again in line with actual police procedure, as a civilian consultant on cold case inquiries.

Complicatingly, in the publishing equivalent of a man coming back from the war to find his wife married to someone else, Rebus bumps into Rankin's new literary partner. Malcolm Fox of the ethics and standards division  hero of Rankin's two most recent police books, The Complaints and The Impossible Dead  is horrified to discover that the cop he regards as "the loosest of canons" is back on police business, and reactivates the "whole shelf" of complaints made against Rebus during his career. Fox's hope is to prevent Rebus from corrupting his former sidekick, DS Siobhan Clarke.

The sections in which Rankin's two characters find themselves in the same book are fascinating psychologically because the author so clearly lets the older man have the better of the exchanges. When Rebus notes Fox "sliming" around HQ and reflects that he seems more like "middle management in a plastics company", it's clearly what Rebus would think about the interloper, but also usefully channels a resentment that Rankin readers must inevitably feel about the loss of their favourite cop. There is also the tantalising possibility that Rankin is employing Rebus to express his own equivocations about the replacement character.

Although their behaviour seems starkly contrasting  Fox sipping Appletiser, Rebus swigging beer and whisky  one sentence suggests that the men are more similar than they, or we, may think: "Fox had ceased to take alcohol because he was an alcoholic, while Rebus continued to sup for the exact same reason." That line links to the Jekyll/Hyde divide that runs through Scottish literature and, indeed, the early Rebus novel Hide and Seek. If there is another double novel, Rankin could usefully make more of this conceit.

Admirers of the Rebus books will be relieved the hero has returned with little change except an increase in the severity of warnings from his doctor. His closest emotional relationship is still with his car  a Saab almost as battered as he is  although there are encouraging signs of further reconciliation with his daughter.

While some elements of Rebus are generic (troubles with drink and women), he is without doubt the funniest among the classic fictional detectives, and his 19th case features some fine one-liners and a satisfying gag involving a bossy colleague's stapler. Standing in Another Man's Grave is a less convoluted puzzle than many of the Rebus novels (the solution becomes decipherable relatively early), but it is Rankin's most interesting book politically, marking a vital stage in the consideration of Scottish nationalist sentiment that has been a constant background to the series.

Although this book has only one direct reference to the prospect of independence, it is steeped in the feeling of a country on the cusp of potentially radical change. Because the disappearances are connected by a trunk route  and relatives and suspects have moved or retired to distant parts of Scotland  the novel has the structure of a road movie, with Rebus repeatedly cajoling his straining Saab around the nation: to Glasgow, Aberdeen, Aviemore, Inverness, Cape Wrath and, ultimately, the Black Isle peninsula in the far north where dolphins are seen in the sea.

Obscure or poetically named locations are a motif in the book. At one point, "Inchnadamph" is flagged up as a potential crime scene by a cop who is accused by Edinburgh colleagues of having made it up to tease them. And, on one of his epic criss-crossings, Rebus notes "signposts pointing him to places he would most likely never visit, such as the Waltzing Waters and Killiecrankie".

Pointedly, during his tour, Rebus is reading a book about Scottish myths and legends; in a novel that says hello again to a beloved character, Rankin seems explicitly to be examining the prospect of Scots saying farewell to their past. On one of his trips to the northern coast, Rebus notes the landscape is unlikely to have changed in a thousand years. Later he is given a riff about the country he comes from: "A nation of 5 million huddled together as if cowed by the elements and the immensity of the landscape surrounding them, clinging to notions of community and shared history."

From this book, my guess is that Rebus  if the polling station is close enough to a pub or distillery  will vote no in Alex Salmond's referendum, with the younger Clarke saying yes. Cheeringly, it seems clear from the final pages that there will be more Rebus books to chart the next stage in Scotland's story.

 Mark Lawson's Enough Is Enough is published by Picador.