First was These Are the Voyages: TOS: Season Three, the latest volume of this heavily-researched series discussing the nuts and bolts of the production of the original Star Trek television series. It goes deeply into the archives of memos and notes to give a different take on what was going on at Paramount. Interesting.
Then, Osprey Weapon #40: The Gatling Gun which goes into detail on the development and use of this weapon. Slightly more technical than I was looking for, and less documentation about its use in combat than I would have liked. Not bad overall.