Profession of Arms by Sir John Winthrop Hackett
Comprehensive overview of warriors from the classical world to the WWII era.
Spy Novels
The Charm School by Nelson DeMille
Cold War chiller about the search for "Mrs. Ivanova's Charm School" where Russian sleeper agents are groomed.
Berlin Game and others by Len Deighton
Agent Bernard Samson is an expert at sneaking across the Wall, but he runs into a fair amount of trouble in his many crossings.
From Russia With Love by Ian Fleming
Fleming pioneered the genre and this is my favorite of the series of classics. Who can ever forget the evil Rosa Klebb?
Ice Station Zebra by Alistair MacLean
The best naval intelligence book I've ever read - a "locked room" mystery, but on board a submarine.