I'll join the chorus of those recommending the DMG. As has been said, there is a lot of stuff in there that isn't in the SRD. I have the DMG II but haven't read through it yet (I have a HUGE stack of stuff that I haven't had a chance to read through yet) so I can't personaly recommend it but I look forward to reading through it.
I do highly recommend a subscription to Dungeon. I have subscriptions to both Dungeon and Dragon. They are both excellent magazines, but for a DM, Dungeon wins with all of the adventures and other DM material found within. It is a complete bargain when you compare the price of a subscription to the price of most other game books.
I have been perusing Green Ronin's Advanced Beastiary lately. As others have said, it is a great book and I really enjoy it. I'd give this one a nod as a good purchase as well.
I liked all of the WotC Monster Manuals. I think I'm in the minority here as there seem to be quite a few that didn't like MM II or MM IV. While they may have some stuff that may not be my cuppa, I think they are still worth getting. However, definitely get MM I.
I was hesitant to get the anything in the complete series, but after getting Complete Arcan, Complete Mage and Complete Scoundrel, I'm rethinking my position. Those three were very good (I had Complete Warrior and lent it to a friend and didn't get it back before I moved - it was interesting too but I like the other 3 more).
I also like the Green Ronin Master Class series and the Malhavok Event books. The Complete Book of Eldritch might was pretty good if you are going to have a magic heavy campaign.
Well, that is enough me too, heh. But don't sell the DMG and MM short because some of the info is in the SRD. You'll miss out on some good stuff too.