Hello Jellyjoker,
It's been a long time since I played (about 10 years) and I use to post on the old CBT forums back then as well. Anyway, I have the old 4Th edition box set from FASA and the Fanpro revised edition Classic Battletech Master Rules (hard bound and everything!). What I'm wondering is, what has changed rules wise in the last decade?
In short the latest BT core rule books condense all the FASA/FanPro/WizKid material into seven books.
Hopefully my summary of the books below is closer to being accurate rather than totally out to lunch.
1. Total Warfare (TW) covers Mech combat between BattleMechs and any combination of BattleMechs, IndustrialMechs, ProtoMechs, Combat Vehicles, Support Vehicles, Infantry, Conventional/Aerospace Fighters, Small Craft, and DropShips.
2. TechManual (TM) is for gear heads covering the construction of BattleMechs, IndustrialMechs, ProtoMechs, Combat Vehicles, Support Vehicles, Infantry, Conventional/Aerospace Fighters, Small Craft, and DropShips.
3. Tactical Operations (TO) expands the rules in TW and the TM rules for support vehicle construction.
4. Strategic Operations (SO) focuses on Aerospace rules for Conventional/Aerospace Fighters, Small Craft, and DropShips. The book also has the construction rules for JumpShips, Warships, and Space Stations with the rules to run them in a campaign.
5. A Time of War (AToW) is the RPG piece.
6. Interstellar Operations (IO) provides rules for combat covering the history of the BT Universe from the rise of the Star League 2300 to the Dark Age Era as of 3150. The rules provide ways to have retro-tech, prototype and advanced equipment. There are rules for the construction several systems which include LAMs, Tripod Mechs, and the Word of Blake super Jump system.
7. The BattleMech Manual (BMM) as has been mentioned cleans up BattleMech combat.
While I like physical copies of books I've only purchased two hardcover and then picked all seven up as electronic/PDF copies. If you are a gear head then purchasing the Heavy Metal BT construction applications would help in building your own stuff.