Rules Cyclopedia is on sale at DriveThruRPG. This game was published back in 1991, long after I had abandoned my Basic D&D campaign. This set of rules brought me back to Basic.
Being a player from way back, perhaps 1977 or so, the concept of "edition" was not real clear. I had started with D&D and moved on to AD&D as it seemed like the expected direction. Transitioning from D&D to AD&D seemed expected, but felt unnatural. When second Edition appeared, I had little concept of what it was. It didn't feel like AD&D that I knew, so I did my best to ignore it.
I had difficulties ignoring 2.0 as Unearthed Arcana seemed to be the first indication that a new edition was coming. Back in the 1990s, it was possible to see all of the various sets, in pieces, on a store shelf and it was very unclear as to what was happening.
My campaign had evolved from D&D to AD&D without regard to the change in setting. Our band of adventurers absorbed new materials and tossed others aside. While I said I was playing in Greyhawk, our shared world was a mishmash of Blackmoor, Greyhawk, Mystara and Hollow World, with Mystara taking the lead place.
When I found Cyclopedia on the shelves of my local Waldenbooks, I was entranced. It expanded on classes and levels while adding a few new spells and most importantly, weapon mastery and character skills. It was exactly what I was looking for. Gone was the one paragraph explanation of skills.
I immediately incorporated it into my hodgepodge campaign with only a few tweaks to make it fit the AD&D rule set. All abilities were generated as per the AD&D methods while character classes of race could either be played as described in AD&D or per Cyclopedia's rules.
Technically, that combo of classes and races vs classes should have been very broken, but as players, we made it work. The RC Druid was a subclass of Druid from AD&D, Mystics became a subclass of Monk. The Racial Classes became the "default class" of those races, as if someone didn't pick a specific class to play.
And we loved it.