The 1979 Advanced Dungeon Masters Guide, authored by Gary Gygax, is the original, and still best, Referee's Toolkit. It is almost entirely setting-neutral, and contains nearly every topic that a Dungeon Master could possibly need to deal with, during the course of a regular campaign.
Certainly, that version of the DMG is not exhaustive, but it gives referees the essential tools to create and develop their own campaign, and provides a myriad of other tools to add color and depth to their world. I recommend every referee own a copy of the original DMG, even if you never play that version of the game, nor ever intend to. It is an excellent resource for any version of D&D, or any fantasy RPG for that matter. And the original DMGs are easy to come by, and relatively cheap.
Several months ago, I purchased and reviewed the Ultimate Toolbox, which is billed as another referee toolkit. That is also a great resource, and I recommend it as well (although the price is somewhat steeper than the DMG).
In addition to those two resources, does anyone have any recommendations for other referee toolkits? I also have the first three Arduin Grimoires, Philotomy's Musings, and the Dungeon Alphabet, which are all handy resources, but I am looking for another comprehensive referee toolkit, to augment my current resources.
Edit: I can't believe I forgot to mention the Ready Ref Sheets by Judges Guild. For $3, as a pdf download, this is the same price as it was back in the late 1970's. Also, see the comments section for some additional referee resources. Kellri's CCD4, in particular, is quite general and covers a lot of ground.