A great RPG book that's not Traveller, but comes in handy for Traveller, is the "BlackEagle/BlackEagle Operative's Kit and Handbook." This was produced by Chameleon Eclectic for "Millenniums End," a modern day techno-thriller game. The company is now folded and the book is out of print, but if you can find a copy I highly recommend you get it.
The Operative's Handbook is full of non-rules information that is useful in military campaigns using small units and in black ops/corp espionage campaigns. Has information on tactics for urban and outdoor environments, sniping, demolitions, breaking into a secure site, setting up and avoiding ambushes, field recon, assaulting a position, setting up a defensive position , how to be a bodyguard and protect a VIP, how to gather evidence -- the list goes on.
For players and refs who have never been in the military, this book is packed with good information you can use in many styles of rpgs.
