It was 1962 and Charlotte Bingham was 19 ¾ when a friend of her parents, seeing her drinking champagne at the Ritz, came over to find out the occasion for such profligate behaviour. She was celebrating, she explained, because she’d finished writing a book.

The friend was a literary agent. A few months later Bingham’s Coronet Among the Weeds was a bestseller. Artfully simulating artlessness, the precocious memoir described her life as an ebullient ingenue in search of a Prince Charming among the “weeds, drips and leches” she met in posh society. It was fresh and funny. Candour appeared to be its distinguishing characteristic. Now a veteran fiction author, Bingham has written that book’s prequel, and we discover how much – as an ostensibly guileless girlish narrator – she was actually holding back.

The father whose tickings-off she had to endure when she crept home late from the Blue Angel club was a senior officer in MI5. When he drank, which he did a great deal, he always held his glass in his left hand, just in case he needed his right to reach for his gun. When Charlotte (or Lottie) turned 18 he summoned her into his study and told her it was time she knew what his “boring work in the War Office” really was, and, moreover, that he was arranging for her to join the “section”. This book is an account of the months she spent there.

Her father, when he drank, always held his glass in his left hand, just in case he needed his right to reach for his gun

During the second world war Bingham père had been one of the instigators of the double cross, feeding false information to German spies. By the 60s the secret service’s objectives had shifted. From her vantage in the typing pool young Lottie watches as her bosses seek for ways of justifying their department’s existence while keeping Britain safe from communism. The former aim is accomplished by exaggerating the urgency of the latter.

In a coda, Lottie imagines future historians finding her MI5 memoir in the public archives in Kew. “No one will believe it.” Actually it’s all too credible. The ex-naval commander so incapable of adapting to the complexities of peacetime that only jam doughnuts can cheer him up; the women from the filing department patiently laying aside their knitting to act as guinea-pigs in interrogation training: these succeed as comic routines because office life (as everyone from Evelyn Waugh to Ricky Gervais has known) tends towards absurdity.

Home is more sinister. It is full of what Lottie’s mother calls “bogeys”. What they’re doing there Lottie never exactly knows. “They’d take their drinks into the garden and mutter, which men seemed to have a habit of doing.”

Two actors come to stay, not only because they are fun – Cole Porter songs around the piano before Sunday lunch – but because “a country can lose its way overnight after seeing the wrong play or film”. Lottie’s father devises a scheme whereby a famous star agrees to appear in a pro-labour play, the British Communist party invests in it, it’s a flop and so “the Trotskyists” are out of pocket. His next wheeze is to employ one of the actors as the frontman of a phony Working Man’s party designed to flush “reds” out of the woodwork.

Quite where our narrator stands in relation to these questionable manoeuvres is hard to judge. In a rarely reflective half-sentence Lottie remarks that she feels a bit sorry for her father when he unmasks a traitor he was at university with. That’s about as far as she’ll go in the way of judgment or commentary. Her preferred tone is sparky and larky. One farcical episode involves a policeman climbing in through a window in pursuit of a suspected burglar who turns out to be Lottie’s latest beau, and is appalled to be confronted by her father waving a swordstick. Bingham is a self-confessed chameleon. “A good way to pass a boring few minutes,” she remarks, is “to pick up the telephone and pretend to be someone else.” As Charlotte Bingham she has 33 popular novels to her credit, but, in this book, she writes as though she were still Lottie – the sharp-eyed but scatterbrained girl who spends Saturdays varnishing her toenails, “which is always such a job”.

Cold war British counter-espionage was often cruel. Lottie’s friend Arabella explains, over a teacake in the canteen, that the service’s purpose is to make socialists’ lives “unmitigated hell. But in a very nice way, of course, because that is what we British do.” This book is frivolous, but in refusing to take spooks seriously it may be as sensible as it is amusing. “You’re in danger of becoming a lightweight,” Lottie’s father tells her. Perhaps it’s cleverer to be light than it is to be ponderously paranoid. Some of what he himself does is in danger of becoming not only morally reprehensible, but plain daft.

• MI5 and Me: A Coronet Among the Spooks is published by Bloomsbury. To order a copy for £12.74 (RRP £14.99) go to guardianbookshop.com or call 0330 333 6846. Free UK p&p over £10, online orders only. Phone orders min p&p of £1.99.