When starting a new book, the first thing is research!
Ronald Kessler’s book ‘In The President’s Secret Service’ was a very insightful look into the Secret Service, it’s history and mythology.
Also, ‘Within Arms Length’ by Dan Emmett has been a useful look at how the Secret Service operate from one Presidency to the next.
It can be hard, trying not to perpetuate assumptions about institutions that are not personally known to a writer, but that should never stop anyone from writing about them.
Research often leads to a more creative storyline that is less stereotypical, and far more unique that in would otherwise have been.