Some years ago on World Book Day I found myself touring around London’s bookshops with a number of other writers. Howard Marks, the celebrated drug smuggler and ex-jailbird, lounged at the back of our coach, making a number of phone calls. “How much do you want? Yes, yes, I can do that for you, no problem.” Chris Ryan was there too.

Andy McNab’s bestseller Bravo Two Zero, an account of an SAS unit’s misfortunes hunting down Scud missiles during the Gulf War, published in 1993, created a new sub-genre in military chronicles: the special forces confessional. An explosion of titles have followed, to the point where we might expect Conan, the dog that barked down Isis’s leader Al-Baghdadi, to be spilling the beans. Of the eight men in McNab’s unit, three died, four were captured, and only one managed to escape: Chris Ryan. He simply strolled out of Saddam’s Iraq, dodging the entire Iraqi army. Even by the standards of the SAS, his escape was unbelievable.

What I found most remarkable about Ryan was how unremarkable he looked. I wondered whethhe’d hired a stand-in to do the dreary stuff of signing his books. But no, it was him. Deputy bank manager ordinary. Average height, average build. You wouldn’t have paid him any attention if he’d been standing next to you at a bus stop, a quality that’s probably a bonus for an SAS operative.

His description of growing up in Walsall won’t do anything for the tourist industry

In the latest volume of SAS memoirs, The Hard Way, Mark “Billy” Billingham, a star of the reality TV series SAS: Who Dares Wins, makes the same point. He was usually the smallest and skinniest guy in most of the challenges he faced, military or otherwise, but he succeeded, by a combination of sheer willpower and wits.

You’d expect a distinguished member of the SAS to be tough and combative, but Billingham was already into fighting and almost getting killed as a teenager, long before he signed up for the British army. His description of growing up in Walsall won’t do anything for the tourist industry.

He almost bled to death after being stabbed, and then nearly died in an industrial accident, working underage at a factory. Stealing a hat led to the young Billingham being invited to a boxing club, which gave him a discipline he relished. Like most boxers, Billingham gets positively lyrical about the subject.

Billingham, who admits he was “shit at spelling”, gives full credit to his ghost writer Conor Woodman, who has done a slick job of packaging together Billingham’s memories and anecdotes. Like many a tearaway, Billingham found salvation in the army, and while his experiences in the Parachute Regiment and the SAS selection process will be familiar to anyone who has sampled an SAS bio or two, they are related in a very cogent manner.

Billingham’s work after the SAS, acting as a bodyguard to celebrities such as Tom Cruise and Brad Pitt, and his cameo role in a Sean Penn film, provides the funniest anecdotes. Waiting in a room full of hulking beefcake bodyguards who regularly protect Hollywood stars, the slightly-built newcomer Billingham is regarded with open derision. After the other bodyguards find out who he is, however, they courteously open the door for him.

Two things are surprising in The Hard Way. First, the scowling, dyspeptic Billingham you see on your television screen contains an inner hippy. Like some gap year idealist, he’s an ameliorist. He may have recruited mercenaries, but he also volunteered to go to a number of difficult, dangerous places to help others. He spent a long time in Haiti after the 2010 earthquake arranging reconstruction, and did a stint in Kenya training anti-poacher patrols safeguarding elephants.

As he explains at the beginning of the book, “During my darkest days it’s humbling to remember there are others in worse situations and usually it’s through no fault of their own. Bad governments, bad people, bad decisions, bad luck. It reminds me to get out of my own head and do the right thing for the greater good.”

The other surprise is that there’s barely a word about Billingham’s time in the SAS. We get a lot of detail up to selection and a lot about his post-SAS career but nothing about slotting drug lords or terrorists. “People might think they know what we’re about,” he writes, “but, unless you have served, you don’t have a fucking clue.”

The fact that Billingham has chosen to remain tight-lipped about his adventures with the SAS might disappoint some readers. But then deception, as Billingham is at pains to point out, is one of the great military skills.