Indeed, I will add to the chorus here: AD&D's Dungeon Master's Guide is a great book for any DM who wants a different take than Monte Cook's (author of third edition's DMG) on DMing, this time by none other than Gary Gygax, "Father Of Tha Game" (TM).

So you get loads of charts... and weird words used here and there... and tiny fonts... and so on and so forth, but through it all there truly is a vast collection of gems to be found.

Don't try to read too much at once, and just flip back and forth through the book by thinking about this or that issue about building a campaign world or dealing with magical items: you will always find a little something that will prove to be inspiring and intelligent somewhere.

I recently purchased this PDF. I already own two hard-copy DMGs, but they are packed away and I found that I need to reference this book quite often. I especially like the section on hirelings, but there is too much valuable information to list.

I totally agree with farewell2kings on this. I still have every book I ever bough about D&D since the early 80's and the 1st Edition DM's Guide is the ONE book I find myself still using after all these years. The random encounter tables (by terrain type) are really great. WoC, if you're looking for an idea for your next book to publish... A COMPLETE BOOK OF RANDOM ENCOUNTER TABLES would be the way to go!

If you play 3.5, you will still find many useful things in this book. If you buy only one 1st edition rule book, buy the DMG. I've had mine since 1981 and it is just a super resource for DM's. I cracked mine open a few weeks ago and it was just surprising how much stuff in there could still be applied to 3.5. Well worth the $4, even if just to take a trip down memory lane or connect with the way the game used to be played.