Mick Herron’s Slough House spy thrillers are, by now, one of the least well-kept secrets in espionage fiction. Everyone with even half an eye on the genre knows he’s somewhere near the top. He is routinely and predictably compared with Le Carré, and it’s a comparison that both holds up very strongly and doesn’t hold up at all. It holds up strongly in the sense that Herron is a kind of burlesque counterpart to Le Carré: his Slough House is a parodically shabby counterpoint to the master’s Circus. But his methods, his style and his tone – these books, unlike Le Carré’s, are halfway to being comic novels – are a thing quite of Herron’s own.

Joe Country is the sixth full-length novel in the series. The deal, for newcomers, is this. There are the proper spies, who inhabit glossy offices in Regent’s Park, report to ministers and traffic in matters of national security, and then there are the failed spies, the cashiered spies, who occupy a dismal building called Slough House, sandwiched between a newsagent and a Chinese restaurant in a dismal backstreet near the Barbican. They traffic in matters of traffic – or parking tickets, or census data, or any other Sisyphean make-work task that can pointlessly fill their days. It’s the latter – the ‘slow horses’, as they’re called, punning on the name of their office – that Herron is concerned with.

If you leave a disk full of top-secret information on the counter at the chippy, or bring a major transport terminal shuddering to a halt because you’ve cocked up a training exercise, Slough House is where you end up. You hope to go back to the Park, but nobody goes back to the Park: the general idea is that the grinding dullness of this Breakfast Club of espionage will cause you to give up and quit the service altogether, or save everyone some paperwork and throw yourself under a bus.

Over this glum troupe presides Herron’s magnetic antihero, the spymaster Jackson Lamb. The extent to which Lamb sucks up the limelight whenever he’s onstage is indicated by the way the books are these days billed as ‘Jackson Lamb thrillers’. Lamb is fat, flatulent, cynical, obscene, stained with the orange leavings of takeaway food and usually in his socks with a Scotch and a fag on the go. But, of course, Lamb is also a deadly superspy who is always two steps ahead not only of his slow horses but also of the high-ups at the Park. He knows where the bodies are buried and how to use those burial sites as leverage to get what he wants. Also, he’s secretly rather honourable: the one thing he always does is look after his joes.

Each novel contrives to bring Lamb’s hapless team, accidentally, into the way of some sort of proper spying, and this both livens things up and – as often as not – sees one or two of them killed off to free up a desk in Slough House for a new character. In this novel, the newcomer is Lech Wicinski, who has been bumped onto the B-team after child pornography was found on his work laptop (as Lamb puts it, ‘He’s already Kevin Spaceyed his career … If he wants to go for the full Rolf Harris he’s a better man than me’). The ongoing story, I should say, means that the novels are becoming less satisfactory as stand-alones. You can enjoy each one in isolation, but new readers really should start with Slow Horses and work their way forwards. This one, for instance, picks up strands from London Rules and the novella The Drop, and brings to a conclusion the story of a long-time slow horse, River Cartwright, and his relationship with his ex-spook grandfather David Cartwright, aka the Old Bastard.

The novels have a satisfying general form. The main story of each begins and ends with an old-fashioned omniscient-narrator passage, in the present tense, resembling the opening of Bleak House. It winds into London, and towards Slough House itself, establishing the bricks-and-mortar setting and its place in the city: ‘The building is a bad tooth set in a failing mouth. Here is where nothing happens: nothing to see here. Move along.’ And then we go straight to the characters – in this case the excellently obnoxious Roddy Ho, Slough House’s pet computer hacker and a tragic loser even by the standards of his colleagues, his loserdom offset only by his titanic levels of self-esteem. Herron’s brisk narration is in the free indirect style, hopping deftly from one character’s head to another. At one moment, for instance, Shirley Dander is thinking ‘time to grab some food’ in response to seeing J K Coe pass her door with a sandwich; by the next paragraph we’re with Coe eating his sandwich at his desk. That handoff is a nice piece of literary tradecraft – a brush contact, you could call it. The one character whose head we never inhabit is, of course, Lamb himself, around whose ‘monstrous calm’ the novels’ world orbits. ‘Lamb’s face’, Herron writes at one point, ‘looked like a candleless pumpkin: its holes and hollows lacking any internal flame.’

Herron is a fine, often glorious sentence-by-sentence writer, and fiercely funny with his dialogue. He plots very well too. Joe Country sees River Cartwright’s dangerous biological father, Frank, back in play, while the teenage son of the late Marcus Longridge has gone missing. Upshot: our heroes spend the last two-thirds of the novel on a violent away day in a snow-shrouded corner of Wales dodging a kill-squad. There’s more action and a bit less of the labyrinthine spook politics in this book than we’ve come to expect – and to my mind it suffers very slightly for it – but it’s still a hugely satisfying addition to the series. And in its dramatic cliffhanger payoff, it seems to promise a portentous new direction. Personally, I can’t wait.