Page 60 of Coup d'État by Edward Luttwak describes a large number of US intelligence agencies, the nominal hierarchy, and (I think) the actual hierarchy.
The NGA is in there, interestingly, along with a bunch of others I never thought existed.
The first edition (1968) is timeless, but the revised edition (2016) is amazingly on-point and is updated to reflect the structural changes which have occurred in the half century since it's first printing. It's only 300-odd pages, highly recommended.
The NGA is in there, interestingly, along with a bunch of others I never thought existed.