Torg originally came out in bits; the core box had the basic system rules, drama cards, and the 'world book' that described enough of each of Torg's realms to get the GM started until the realm sourcebooks came out. (Also an adventure book with a starting adventure.)
Realm sourcebooks provided details on the different realms and also completed the rules, one topic at a time. (For example, the magic rules were in the sourcebook for Aysle, the fantasy realm, and the weird science and pulp fiction superpowers were in the Nile Empire sourcebook.)
The book I think you're referring to is the Torg Revised and Expanded rulebook. This rulebook was assembled by Torg's most dedicated fan-writer, Jim Ogle - basically it's all the core rules assembled in one place, with some editing and tidy up, and is effectively "Torg 1.5". However, this rulebook is a core rule book; it doesn't contain the world source material, or the drama deck - it was intended as an update rulebook for existing GMs, not a replacement for the core box set. To go with it there was an 'Accessory pack' which included everything from the original core box set except the rulebook.
The game uses a 'drama deck' of cards, originally available only in the core box set. The rules can be made to work fine without it, but a big part of Torg's strength is the way that the deck encouraged unconventional combat actions and created player control of the plot.
The deck was later sold separately so you may have luck finding it independently. For a while it was available on RPGNow in PDF, but that seems no longer to be the case. You can however still get the drama deck from the successor game system, MasterBook; this is similar enough that it can be made to work OK. (For people who use FileMaker, there's an old Filemaker drama card database that will print the cards.)
So, short form:
You could run Torg from just the core box set, or the Revised and Expanded rulebook. But you probably shouldn't, because you'll be missing most of the strengths of the drama deck and the cross-genre realms, and without those there's no reason to play Torg at all. (At that point, Feng Shui would be a better choice.)
To run Torg well, I'd recommend a minimum of:
- Either the original core box set or the revised and expanded rulebook with a separately bought or home-printed drama deck.
Plus at least any two of:
The Aysle sourcebook (The fantasy realm and magic rules.)
The Nile Empire sourcebook (The pulp hero realm with weird science and pulp power rules.)
The Cyberpapacy sourcebook (A fun cyberpunk medieval-church realm, with the cybertech rules. Detailed netrunning rules were in the GodNet sourcebook, which is optional unless your group has a decker character and will spend a lot of time in the cyberpapacy.)
The Orrorsh sourcebook (The Victorian horror realm and monsters.)
The Kanawa sourcebook (Modern-day cyberpunk corporate espionage, with extra ninja and the martial arts rules.)
Rules-wise the Aysle sourcebook was originally the most critical, as several realms have some form of magic, but there's a bare-minimum spell list and magic rules in the Torg R&E book.
Good luck. Torg is hard to find now but well worth the effort; I'd argue it was the most creatively forward-looking RPG of the early 90s, implementing very early many of the great ideas that RPG design now takes for granted.
(In particular: Torg did cinematic combat before Feng Shui, multiple game character styles on an integrated system before World of Darkness, player control of plot before just about anybody, and a global integrated metaplot affected by local groups' results before... well, actually, nobody's managed it since. LFR is fun but doesn't come close.)