The Enigma of Kidson is a quintessentially Etonian book: narcissistic, complacent, a bit silly and ultimately beguiling.

It is the story of Michael George MacDonald Kidson (MGMK, as he was known), who taught history at Eton from 1965 to 1994 and was an influential tutor to hundreds of boys, often the wayward and the damaged.

Jamie Blackett, who was taught by him there, has collected Kidsoniana from former pupils, colleagues, friends and acquaintances. What emerges is a portrait of a colourful maverick who bullied and consoled generations of schoolboys into success and happiness.

Blackett conjures up a cheerful world where robust and affectionate Springers (Kidson’s Dougal, Boody, Bertie, Charlie, Jed and Faddy) and Etonians (who, from a certain point of view, now seem to run the country) lollop around, getting into scrapes and having fun. MGMK’s pupils include the former prime minister David Cameron, the current Archbishop of Canterbury Justin Welby, at least one Catholic priest, at least three Olympic medallists, umpteen politicians, uncounted peers, racehorse trainers, actors, bankers, writers, lawyers and businessmen, as well as a clutch of convicted criminals (one pupil went straight from school to borstal and later became Chief of Police in the Dominican Republic).

At Kidson’s funeral, Nat Rothschild described his miserable first two years at Eton. Then he met Kidson, and life changed. ‘I looked forward to his lessons, his humour, his rages. The way he taught us lightened up my day.’ The sensation that he illuminated, even redeemed, teenagers permeates Blackett’s book. ‘He made us feel we could walk on water,’ Tom Goff remembers.

The Enigma is full of stories of Kidson’s theatricality. His insults were legendary, but his pupils seem to have understood the histrionics and loved him for them. Many recall him hurling half a croquet ball or a board-rubber at them (‘Kidson, Michael: choice of missiles’ is an index entry). According to Blackett, boys knew instinctively that he was on their side and that he would do what he could to support them against the forces of the adult world. Father Alexander Sherbrooke, who runs a parish in Soho, learnt from Kidson that if someone asks for help ‘you must leave no stone unturned to help them’.

We also hear about Kidson’s slightly tiresome snobbery and pernickety pronunciation. Goff, westcuts and cumbut pepper the text, a dash heavy-handedly, and Blackett eggs the pudding. When Michael Cecil’s entrance is footnoted the reader knows, with clanging foreboding, that the footnote will explain how to pronounce his name.

However, although Kidson’s pupils were devoted to him, few knew anything about his early life; he had a tendency to sidestep questions. Blackett has discovered that he had a precarious childhood. By the time he was three his father had run away to South Africa, returned to England and spent six months in prison. Soon afterwards his rackety mother ran away — never to be seen again — and Michael went to live with his grandfather. At 16 MGMK was alone in the world, his father and grandparents all having died. After National Service and Cambridge he did a stint with Shell before going to teach at a prep school. In 1965 he found his niche when Anthony Chenevix-Trench offered him a job. Kidson flourished at Eton and his mysterious past became part of his increasingly flamboyant persona.

His success as a teacher (a ‘beak’ at Eton) lay in his ability to bring history to life. He focussed on heroic individuals. Gladstone was in Kidson’s mind the greatest of the great men of the 19th century. Cameron confirms a story that he had tears streaming down his cheeks when reading the account of Gladstone’s death in Philip Magnus’s biography. Palmerston and Peel were also great statesmen, in Kidson’s view; Disraeli a mountebank. Kidson once set an essay — to which he expected reasoned and rigorous answers — entitled ‘How Greasy was Disraeli’s Pole?’

Tucked away in The Enigma’s end matter is material that suddenly explains all the encomiums of the preceding 306 pages. Kidson’s lecture notes for classes on the monarchy and the Victorians show how compelling his divs (lessons, at Eton) must have been, and — therefore — why his pupils wanted to learn. Read The Enigma of Kidson for these if for nothing else.