As fascinating companion to any reading of British history through the 19th century, for example Heaven's Command by Jan Morris.
Through the lens of the evolving intelligence service, we get a fresh take on events in India, Middle East and especially relations with Russia. Where other books can inform us of what happened, here we glimpse much more of why and how from a behind the scenes and more strategic perspective.
The book appears thoroughly well researched, and only able to bring new facts to light now because many of the source materials were previously unavailable to most historians.
And unfortunately for the book, it's readers and history, those sources turn out to have definite limits. As the author admits, there remains much that cannot and will never be known.
I felt this particularly in relation to operational details. There are delicious hints towards an evolving operational capability in espionage and counter-espionage - a highlight in the 'dirty tricks' category being falsely identifying a "person of interest" as a wanted criminal and leaking this to the press in order to keep them out of the country. However it is sadly rare that we get to find out much more than what was summarized in a diary or report that somehow survived successive purges of secret and sensitive materials.