I read also the Dungeon Master’s Guide after reading the Player’s handbook. The Second Edition put most of the rules in the PHB, contrary to what the First Edition AD&D did, so this is a thinner book.
In addition to being thinner, the DMG has some duplicate information from the PHB. I have no idea why, as I think every Dungeon Master has also the PHB. I think the largest single section is the duplication of unarmed combat, which is quite strange, because I think nobody really used it.
In addition to special rules for situations, like more combat rules (flying, morale, and things like that), much of the book is dedicated to game mastering advice. There’s talk about alignments, classes, races and of course the dreaded class generation rules. They looked so bad even when I was thirteen that I didn’t really try them out ever. The gamemastering advice (or dungeonmastering advice, more properly) is okay, but not great. The alignments are of course an important thing, but there is one strange thing: the advice says that characters should never be sure of each others’ alignments. All fine, but the PHB has the second level wizard spell “Know Alignment”…
Of course forbidding the spell does much to the game – now I would run a game where alignments are not so obvious. Even then the advice is a bit strange.
There is some advice on how to run a game world, how time is measured and so on. This is also passable – I think I used at least the titles for some rulers and the NPC personality generator many times.
After the advice most of the book is taken by the treasure lists. There really are many different magical items and different kinds of treasure hoard types. Rolling them was fun twenty years ago, but now I’d again probably use some other method for assigning treasure to monsters. Also the Dragon magazine article about the sizes of cooins made me think of the dragon treasures: even ten thousand gold coins isn’t that big a pile.
All in all, not a very good book. It was the first English language roleplaying game I bought, so it had a lot of influence on my early gaming, but looking back now the advice could have been much better. This really isn’t even needed for running an AD&D game – most of the rules are in the PHB anyway, and making up treasures is fun even without a book.