I believe that AToW's (and the companion) list of equipment encapsulates most, if not all, the equipment from 3rd edition (CBTRPG). Minus stuff that was super redundant of course, I can't remember what exactly on hand. If it was for the RPG, the scale of game dealing with one person, AToW and the Companion will cover 95% of it. Handbooks for the Houses and the Major Periphery States still have one or two little things not in those two books.
Combat Equipment's contributions were sifted through AToW and Tech Manual, due to some items being used by support vehicles/Battle armor which is where the latter book comes into play.
Combat Operations, parts went to Strat Ops (How do you wage a big war? Here's some write ups) while the rest went to Interstellar Operations (How to make a bigger war than a Strat Ops version? Here's some write ups!).
They also took that monsterous Inner Sphere in Flames (*shudders*) and put a fixed version called Inner Sphere at War within IO.
At the moment the rule set is geared to 3025, but I'm betting that other eras will follow somehow when the book is no longer beta.
The completionist in me says that any book can be used for RPGs, but whenever there's a conflict in print newer trumps older most of the time.
For the RPG player; I'd suggest AToW, the Companion, and the Handbooks that were recently released.
From there you have at least a good assortment of history, fluff, up to date rules, and potential hooks for fledgling RPG sessions.
The older stuff like Guide to Covert Ops, is nice, but the AToW Companion took over in terms of the equipment.
Guide to the Clans might be useful for establishing a base idea of Clan culture, some of the adventure can be converted to AToW as well.
I'm under the impression that Fanpro was more focused on the RPG, while Catalyst releases very little ATOW stuff, besides fluff books.
Catalyst has released
Lance Pack Dossiers, small lances with RPG stats, and hopefully will help get more AToW stuff out the door with upcoming releases.
http://bg.battletech.com/books/upcoming-releases/Touring the Stars, Milestones, maybe even Gladiator Gazette but those are future products at the time of this posting.
I hope this helps.
Edit:
Oh also TRO Vehicle Annex is a nice one to get, support vehicles and such for RPGs.
I recommend the Revised version.