For tactics training, I highly recommend "1001 Winning Chess Sacrifices and Combinations" (http://www.amazon.com/1001-Winning-Chess-Sacrifices-Combinat...) and "1001 Brilliant Ways to Checkmate (http://www.amazon.com/Brilliant-Checkmate-Chess-lovers-libra...). Both books are things you can toss in your bag and pick up when you have a spare minute.