"Never Unprepared" - well written and organized. The author, Phil Vecchione, is a professional project manager and major RPG fan and this book shows it. It is great for beginners with professional level advice. The advice, as professional as it is, is easy to follow.
The Lazy DM books by Mike Shea (a.k.a. Sly Flourish). I would have thought Tappy wrote this, but the fans know Tappy does not have the time to write a book. For game prep take it is easy; it is just a game afterall.
Rifts Adventure Guide by Kevin Siembieda - Yes, that Kevin Siembieda. One of the vangaurds of the RPG community gives good advice.
While "Never Unprepared" is RPG generic, the Lazy DM books are D&D oriented, and the "Rifts Adventure Guide" is Rifts/Palladium leaning, the advice in all these books can be used in any game. The interesting thing to me is how the same points will reappear in all three books...and they agree with so many Happy Jacks recommendations.