Long-time readers of this blog will know I'm not a huge fan of the 1983 Frank Mentzer-edited version of Dungeons & Dragons, even though I retain a lot of fondness for the Companion Rules that are part of that series. The reasons for my dislike are partially esthetic and partially philosophical, but they're not really important right now, because I'm not actually going to talk about those boxed sets in this entry. Instead, I'm going to talk about their 1991 Aaron Allston-edited descendent, the Dungeons & Dragons Rules Cyclopedia.
The Rules Cyclopedia was, in many ways, the final gasp of OD&D, as it was the last time TSR published a version of D&D that wasn't a sub-set or spin-off of AD&D. Weighing in at just over 300 pages, it was also a complete game, containing all you needed to create and play characters from levels 1 through 36. When I say "complete," I'm not exaggerating (much), since the Cyclopedia had rules not just for characters, combat, spells, monsters, and treasures, as you expect, but also rules for stronghold construction, domain management, mass combat, and planar adventuring -- the entirety of the now-missing D&D endgame. With this book alone, you have everything you'd ever need to play endless D&D campaigns.
Some would assert that the original three little brown books are just as complete as this 304-page tome and do so in the span of far fewer pages. There's much truth in that assertion and goodness knows no one needs as much explication as the Cyclopedia provides in order to create and maintain a D&D campaign. However, the truth is that OD&D, despite being first chronologically, is probably the most advanced of all the D&D rules sets. It's not a good fit for beginners or for gamers who aren't accustomed to having to "wing it" at nearly every turn as their campaign unfolds. For such neophytes, books like the Rules Cyclopedia are a godsend, providing just enough detail to be useful as references but not so much as to reduce the whole endeavor to an exercise in bland number-crunching. The Rules Cyclopedia in my opinion strikes a fairly happy medium between the extremes to which roleplaying games can fall prey.
More than that, though, reading through this book, I don't get the sense that it was part of a grand plan to sell more products. Instead, it feels very much like a stand-alone product. Certainly there are many references to other D&D products -- many of them AD&D 2e products, oddly enough -- but these references don't detract from the overpowering completeness of this single volume. When one compares it to the bulk of TSR's output in the early 90s, most of which were clearly intended as mere pieces in a larger product line, the Cyclopedia seems all the more anomalous. It's as if it were someone's pet project that they somehow managed to sneak on to the release schedule in between Volumes 632 and 633 of the Encyclopedia Magica and after the 115th domain book for Birthright.
That's not to say I love everything about this book. Even as relatively slim as it is, it's still too long for my liking and wastes a lot of space on things I'd rather not have seen (like a skill system or an overview of the Mystara setting). Nevertheless, I always keep this book close at hand and enjoy re-reading it every now and then. When I do so, I often have to fight off the urge to start a campaign using these rules as-is. There's just something very appealing about this book, something that's very hard for me to quantify. I suspect it's that the Rules Cyclopedia somehow manages to steer itself between the Scylla of AD&D's complexity and the Charybdis of OD&D's sketchiness -- a sweet spot for many gamers, especially in an age when modern editions of D&D dwarf even 1e in length and complication.