This is a biography of Vernon Kell (1873-1942), founder of MI5 and its director for more than thirty years (rather than a history of MI5 per se). It was compiled by his widow in the late 1940s, but this account has only now been brought to light by his great granddaughter, Caroline Coverdale.
MI5 is only a part of the story: the first half of the book is a (to me, fascinating) travelogue documenting the newly-married Kells' journey across Canada to China (and back via Siberia) where the polyglot Kell was to add Chinese to his quiver of languages, they arriving just in time for the Boxer Rebellion in which Kell became involved. There are walk-on parts for all sorts of interesting people such as Theodore Roosevelt and the brother in law of Herbert Hoover.
We move on to the MI5 part of Kell's life, which contains some interesting details of particular successes that became public knowledge, but we also learn that Kell never discussed en famille what was going on in his shop. Between the wars, in spite of Kell and MI5 being busy as bees, there is, for the general reader, a bit of a longueur about social matters - the Kells seem to have found hospitable friends all over the world - but one has to remember that this account will have originally been written primarily for the family.
Kell himself comes out of this as very clever, very musical, a brilliant natural linguist, and a born diplomat with the ability to get what was needed out of the System, a gift for handling foreigners of all sorts on the other, and natural powers of leadership. With all these assets it is also clear that he was actually liked, which doesn't always go with those. A lifelong martyr to asthma, he died (in March 1942) rather soon after being pushed out by Churchill in June 1940, by which time he really wasn't healthy enough to carry the strain of the job through another world war.
There are thirteen illustrations culled from Constance's albums, largely about China in the 1900s; I only wish the publishers could have found space for more.
MI5 itself was the subject of an authorised history in 2010