Five years reading the OSR blogs, though it changed my outlook on games radically from the start, hasn't really made me a better DM. So, this year for both my home game (N1 Against the Cult of the Reptile Gods using B/X rules) and my new gig DMin' D&D Encounters at the game store (Scourge of the Sword Coast using D&D Next), I've taken myself to DM School using a quaint old-fashioned technology called PDFs. I was going to say 'books' but they've pretty much all not been actual books.
Anyway, the best book of the lot is Gamemastering by Brian Jamison. Free in PDF. I think the character creation chapters are pretty kooky (mainly because I've only played dungeon crawls that don't involve much in-character roleplaying), but the rest of the book--and I'm not joking now--really does provide a system whereby you can let your players go wherever they like and do whatever they want and you will know what to do when they get there (with a perfectly manageable amount of prep, that is). Maybe you already know how to do that, but I didn't. Maybe you'd like more specifics, but the whole book is free for you to read, so I would direct you there.
The next best is Robin's Laws of Good Gamemastering, which I get the impression is regarded as a classic. It mostly deals with learning what your players what out of the game. Seems like I should have thought of that long ago, but I never did until I read this book. I've seen his system repeated a few times in my reading, so that's another reason I think it's a foundational text. Laws has become my hero, along with Ken Hite (even though Ken can get a little Ted Nugent-y) because of their endlessly intelligent podcast. Ken & Robin Talk About Stuff. It could be the endless intelligence starts right after the name of the podcast.
Laws's Hamlet's Hit Points was also eye-opening, if a little dull to read in the beat-by-beat breakdowns.
Dungeon Master for Dummies (the 3.5 version) was also worth my time. The most important thing I took from it was the emphasis on bringing excitement to your narration and such. I'm pretty inhibited and hung up about expressing myself, so that could result in being boring to my players. Don't want that. This book also has some really neat encounters in it.
And now just a few bullet mentions:
The Lazy DM was succinct and all lean muscle. And the title is after my own heart. I want to read Sly Flourish's DM Tips too, but I'm wary of 4e focus I whiff in the air. I'm not edition warring here, it's just that I won't be playing any more 4e now that I need to learn Next. Well, Next is so much like a retroclone that the learning has been quick.
Play Unsafe was so great. It led me to Keith Johnstone's books about improv, and those are expanding my mind in areas beyond DMing.
I'm also working through Johnn Four's Faster Combat "52 Week Course." It's been released as a book now too. It's probably not necessary. I though I was going to be running 4e in two-hour blocks, so it seemed that speeding up combat would be essential, but with D&D Next, it's not an issuse.
Finally, I would avoid these:
My Guide to RPG Storytelling by Aron Christensen. It's a bad book.
If anyone has read Gamemastering by Dominic Wasch or XDM X-Treme Dungeon Mastering by Tracy Hickman (or anything else on the subject) I'd love to hear your recommendations or warnings.
I guess if I really wanted to know I'd use the internet . . .