With the advent of December and the heart of winter, the holiday season is upon us. You may celebrate Christmas, Channukah, Kwanzaa, Yule, or even just a non-day-specific appreciation of surviving another year. Regardless of the occurrence, if you're a DM or GM deserving of a little extra appreciation, may I suggest you gift yourself the venerable Advanced Dungeons and Dragons (1st Edition) Dungeon Masters Guide.
This is, quite simply, my favorite RPG rulebook of all time, and if you have it in your collection, you probably know why. It's not for the rules -- those are antiquated, clunky, and absolutely brutal (as demonstrated by the sample of gameplay where one of the characters, in the party's first enemy encounter, is instantly devoured in a surprise attack by ghouls following one bad die roll).
What makes this book great, in my opinion, and deserving of a spot in every GM's collection, regardless of what system they actually run, is how it speaks to the experience of being a GM, not just in rules, but philosophy. Every section discussing various rule sets (and there are a ton of them) explains not only the numbers and processes but the rationale behind them. These explanations make much of the book surprisingly rules-agnostic and malleable for GMs skilled in their own games.
This is clearly a work of passion and, regardless of what anyone may think of Gygax and Arneson as game designers, their early work such as this is overflowing with creativity and style. While it is true that an editor could have been used to trim some extra fat here and there, this is perhaps the single most diverse gaming book I've ever possessed, with everything from the magical properties of gems and flowers, detailed (yet non-codified) discussion of physical and mental illnesses, and a lengthy explanations of medieval occupations which adventurers may hire. There are rules for creating magical items, a dice-rolling list for mixing potions together, extensive tables for the powers of mighty magic items, and an incredibly cool list of the ways to destroy artifacts (to this day, discussion remains about just what a "humble ant" is and how its foot could crush an ancient relic.) It is 230 pages of tiny-fonted information.
(It also, after 45 years, still provides the single best explanation of the concept of hit points in a role-playing game that I have ever seen, and one I refer back to when trying to get players to understand why their warrior might sustain 10 attacks with a longsword while the commoner on the street dies after one stab of the dagger.)
Despite its age, copies can still be found for $25-35 in many places, although prices are slowly starting to creep up after all these years. If you can't afford this (and believe me, I understand), scans of the book are also available online in a number of places. Though it's not the same tactile experience as flipping through the book, you can still absorb this old, sometimes off the wall wisdom. Treat yourself, you deserve it.