Its hard to know what to advise without knowing what you have or what sort of advice you are wanting.
One of my favorites is the Villain Design Handbook from Kenzer and Company. The Pathfinder Gamemastery Guide has some good stuff in it, and I've heard some good things about the 4e DM's book. The AD&D DM's Guide by Gygax is an old staple still worth reading, and then for a slightly different take, you can read some of the old World of Darkness books on a more story-telling approach to running a game.
Also, Green Ronin's Advanced Gamemaster's manual has some interesting stuff in it.
The best thing a DM can do though is simply run games and practice trying different things to get a feel for what works for you. Nothing beats actual experience.