Flexor the Mighty! said:
Dm orientated...
I'm not doing all the splats right off the bat...
Advice books are probably less vital than books with must have mechanics or monsters.
First, given the appraoch you set forth above, I'd say that there are no must have books that fit into that framework.
That being said, there are plenty of good books out there to help your games. As to what I'd suggest, I'd say it depends on what you want to run. Do you have an idea?
Without a specific direction, I'll float the following suggestions, based on your stated intention of acquiring a DM focused book, and not getting the splat books, and having your DM focused book lean toward mechanics instead of GM skills advice.
Monster books:
A Magical Society Beast Builder by Expeditious Retreat Press, or Monsters Handbook from Fantasy Flight
Planar:
Manual of the Planes - DM type crunch
Running a certain style of game
Heroes of Battle to run a war type game
Power of Faerun and/or Magical Medieval Society can really help flesh out the backdrop. Power of Faerun specifically is packed, and I mean packed with ideas and adventure themes that are hinted at and left for you to flesh in so far as they interest you.
DMG2
Has a mix of some things, but none of it really essential. For variety though, I think it tops the list. GM skill advice, a premade city and info on developing cities, more magic items, etc.
A campaign setting (if you like to make your own adventures) or campaign module (if you like something pre-made to get the ball rolling). For my money, I'd say Ptolus or Red Hand of Doom, respectively. If you want a setting that costs less money, I'd say just find one with the theme you want to run.
You also can't go wrong spending the money on a subscription to Dungeon magazine. Stand alone adventures, adventure path linked adventures, GM toolkit, maps, pre generated NPCs and stats (even if you don't use the adventures). You'll probably get the best combination of GM resources here as opposed to anywhere else.