The United Kingdom's domestic counter-intelligence and security agency, most commonly known as MI5, was founded in 1909 by Sir Vernon Kell KBE. Kell ('K' within the agency) not only founded MI5 but was also its Director for 31 years, the longest tenure of any head of a British government department during the twentieth century. Kell was also fluent in six foreign languages, making him arguably the most gifted linguist ever to head a Western intelligence agency.

A Secret Well Kept was written by Kell's wife, Lady Constance Kell, in the late 1940s. It is an unparalleled insight into the personal life of an extremely powerful and important man, from the one person with whom he was most intimate.

Nearly half of A Secret Well Kept is devoted to Kell's life before MI5 and about half of the section on Kell's life from 1909–1940 covers his personal life outside MI5. We also learn about the key characters and events during Kell's career with MI5--particularly the key people he worked with and the big spy cases from 1909–1918. While there have been numerous studies and histories of the work of MI5 already published, there have been none of such an intimate nature as this and the primary value of A Secret Well Kept is that we learn a great deal about the background, talents and character of the man who founded MI5 and directed it for over 30 years.

Lady Constance Kell was Sir Vernon Kell's wife for over 40 years. She wrote A Secret Well Kept after his death in 1942.

An introduction to the biography is written by Dr Chris Northcott. Dr Northcott has taught Intelligence and Security Studies at the universities of Salford and Bedfordshire. He wrote his PhD on the development of MI5 from 1909 to 1918 and he has been published in the International Journal of Intelligence and CounterIntelligence. He is author of MI5 at War 1909-1918 (2015).