I picked this up because it was an Amazon Kindle Daily Deal, and ended up rather enjoyed reading it. It is dry most of the time, focused on facts, procedures, and events that happened mostly in an impersonal way, although it has some fairly candid photographs of training and other activities taken by the author (and other illustrative photographs from other sources).
That presentation also conveyed a lot of useful information (albeit somewhat repeated, probably especially in the Kindle Edition). It is the sort of book to read to understand the Secret Service's history, big events that shaped it, what its responsibilities are, how their operatives are hired and trained, and the divisions and teams within it and what they do.
It strikes me as the book I'd return to if I were wanting to make up a character who is in the Secret Service - the sorts of backgrounds the operatives have, the trials and training, and the different duties they may have had over some time there.