From AD&D onwards, it's been the Player's Handbook/Dungeon Master's Guide/Monster (Monstrous) Manual trio. A clear separation of the players' materials from the DM's material, with the monster info separated from the world building/running aspects.
The Essentials line for 4ed D&D has introduced the Rules Compendium, a separation of the rules from either the player's or the DM's side. Unlike 1991's Rules Cyclopedia, which had class crunch, the Rules Compendium has no race or class crunch in it. Unlike the 3ed Rules Compendium, the 4ed Rules Compendium was meant to be released at the start of the edition (or, in this case, the start of the Essentials line).
I really like what the Rules Compendium represents as a starting place for an edition of D&D. Before, the rules for the entire game were split between two books that not all players were "allowed" to look at. The modern trend in 3ed and 4ed to demystify and publicize the rules is a good thing to ensure DMs don't abuse their power. If the next edition of D&D starts off with a Rules Compendium, there are definate benefits.
Any errata added to the underlying rules don't require the books for players' options to be changed. RPGers would know better the effect of errata: changes done to the Rules Compendium would reflect the changes to the rules, and changes to the Player's Options books would reflect the changes to specific races and classes.
An added benefit would be that the Rules Compendium could be a starting point with any future Player's Options book. Let's assume that the Rules Compendium has everything that every class would need: armour, non-exotic weapons, adventuring gear, etc. The edition's first Player's Options book could be as compatible as a launching point as the second, third, or fourth. This would help allievate the bloat of long-lived editions, if there were less required reading. Right now, a lot of posters warn new players to avoid the PHBI since the rules are heavily modified: a PHBI without rules could stand on its own merits.
What I would like to see at the launch of 5ed is:
- The Rules Compendium, containing all the rules players need to run their PCs, and all the rules the DM needs to run the game.
- A Player's Options book for the races and classes
- A Monster Manual book for the monsters
- A Dungeon Master's Guide based solely on guiding people through the task of DMing, troubleshooting problem situations and players, and inspiring them for better games.
- A new version of the Player's Strategy Guide that does what the DMG does for players.
I'll get into what the 5ed DMG and PSG would look like next week, but for now, I want to wrap up my thoughts on the Rules Compendium. If WotC did expand the Core books from 3 to 4 or 5, there would be outrage. Gouging, insulting our intelligence, etc. However, I think that we would receive better quality of the books we buy. Less rules in the Player's Options book means more crunch for classes. Less rules in the DMG means more material to inspire the most important person in the group.
From what I've read on the forums, the Rules Compendium has been warmly received by many people for filling a niche (a small-yet-comprehensive book of all the rules you need to look up during the game). If it's part of the initial line-up, it can play that role on Day One, and, if needs be, can be replaced with an updated version with less impact on the first Player's Options book.