I remember buying this, even though I still had my B/X rulebooks and could borrow the rest of the BECMI sets, because I wanted to have one solid reference -- with monsters -- in my hand whenever I wanted to run this game.
Once again, the entry's description has tidbits of historical significance, courtesy of Shannon Appelcline. Here's an excerpt:
"The Compilation. The Rules Cyclopedia is a compilation of the D&D Basic Rules Set (1983), the D&D Expert Rules Set (1983), the D&D Companion Rules (1984), and the D&D Master Rules (1985). It contains not only the rules from those boxed sets, but also the monsters, making the Cyclopedia one of two great sources for Basic D&D monsters, the other being the Creature Catalog (1986, 1993). Approximately 150 pages of rules on characters and magic from the "GAZ" Gazetteers (1987-91) are also included, making the Cyclopedia a truly massive compilation of about a decade's worth of Basic D&D rules.
The Immortals Rules (1986) are notably not included in the Cyclopedia, although it does contain seven pages from the Master Rules that include basic information for immortals, including rules on PCs ascending to those lofty ranks.
Rules on jousting in tournaments and on artifacts were also left out of the Cyclopedia.
Not an Introductory Book! Unlike every other iteration of Basic D&D, this one was not intended to be an introductory roleplaying book. It was instead a reference for Basic D&D play, which matched TSR's thinking about the AD&D 2e rules.
Expanding the Known World. The Cyclopedia doesn't expand the Known World in any notable way, but it does include a rather impressive atlas, featuring 16 full color maps, including the maps from the Gazetteers and the world maps from the Master Rules and the Hollow World Campaign Set (1990)."What are you waiting for? Pick it up now!