What do you consider to be the essential package for a given Game system; that is to say, which books would you consider as basically essential to running the game?
Obviously, the question only applies to multi-book game systems.

In my opinion, for D&D only the three basic books will fall under that qualification. There's no other book I consider so useful in the game that its vital to be used.

For Warhammer, though, I would say that you'd need the main book, the bestiary, the Armoury, and Realms of Sorcery if you have a wizard in your group at all.

RPGPundit

Call of Cthulhu, the rulebook.

Pendragon, the rulebook and Lordly Domains.

Traveller, the three little black books.

Gurps fourth edition, the rule book.  Gurps fifth I have no clue.  Edit:  Please read that as third and fourth respectively, I got my edition numbers mixed up.

I tend to like one book games.

The new Mongoose Runequest, about a dozen books as best I can tell.

There is no gurps 5th yet. As for me and gurps, I'd consider the two basic books along with ultratech, assuming it ever comes out, to be essential, along with maybe a good SF setting and/or biotech.

I like one-book games too.

Otherwise it's just an attempt to get you to pay 4 or 5 times as much for a set of rules as you would normally, as with the Mongoose version of Runequest.

Quote from: Mr. AnalyticalI like one-book games too.

Otherwise it's just an attempt to get you to pay 4 or 5 times as much for a set of rules as you would normally, as with the Mongoose version of Runequest.

Quite, the way to sell me extra books is to make them so cool that I want them, not to make them necessary for me to play at all.

Quote from: BalbinusGurps third edition, the rule book.  Gurps fourth I have no clue.

Well, for GURPS 4th, all you really need is the Characters book to play, but to run it you'll need the Campaigns book as well.  Powers is nice, but not 100% necessary unless you're running something that isn't just normal people.

Quote from: Mr. AnalyticalI like one-book games too.

Otherwise it's just an attempt to get you to pay 4 or 5 times as much for a set of rules as you would normally, as with the Mongoose version of Runequest.

I agree, I also like one book games.  It's one reason I've played a lot of the Forge stuff...one book, and that's it.

And I agree also with Balbinus, make the sourcebooks unnecessary but awesome.  In Nomine springs to mind.  I've only ever needed the core book, but some of the supps were cool enough that I bought them.

I'd be loathe to run Traveller without Supplement 4: Citizens of the Imperium.  Mekton Zeta only works for me if you have the MZ Technical Manual to go with it.  I could run Pendragon without the Boy King, but I'd rather not if I had the choice.  Star Frontiers without the Knight Hawks expansion is kinda sad.

Quote from: RPGPunditIn my opinion, for D&D only the three basic books will fall under that qualification. There's no other book I consider so useful in the game that its vital to be used.

We tended to limit it to only those books as extra books tended to be counterproductive in some ways.  Certainly from the DM's seat it be all you really need. On the player's side of things it easily is too if your DM has flexibility in allowing custom created Feats, PrC, Spells, and so on that you would normally find in the splatbooks. While this does involve some extra work in creating those things, you also do get what you want instead of hunting through numerous books trying to find the best fit for what you want and what'll work in the game as a whole.

EDIT: Unfortunately the marketing and mentality is strong on requiring the extra books for the crunch of the extra Feats, Monsters, and such. Because that's how WotC feeds the corporate beast.  So it can be a tough sell getting some DMs into the flexibility of introducing something that isn't published in a WotC book, even if they actually go through and further vette all those individually before accepting a particular one.

For SR3 it was the core book plus the core splatbooks. You could sort of run with just the core, and there were/are a few that do. It is technically possile. But the rules that people actually used tended to be is the splatbooks. Well the rules people actually used tended to not be in any of the books, but that's another issue.

For SR4 they moved a lot more rules into the core book. We'll see how all the splatbooks pan out. So far there is just Street Magic and it is really, really cool but you can run a fairly extensive campaign without out it. I suspect the cars, guns, and implants books are going to be the same deal.  The Matrix splatbook Unwired is likely the one that will break that trend as that is the weakest section of the core book. So it seems in SR4 that running with just the core book is a lot more viable. They still don't have the methods for building the contents of the splatbooks within the core book though.  That's something that I really like to see in rules now.

EDIT: I will toss in Burning Wheel here, since noone else mentioned it.  For games that are humans or even demihuman centric if you like Toilken's feel for elves, dwarves, and orcs the core book dual (they come strapped together in the store, you can only purchase them together, and at a one RPG book kind of price) of BWR and Character Burner will get you through.  If you want to branch out from the default Toilkenesque setting and into other settings (even outside fantasy) then you'll likely want to pick up the Monster Burner, the only other BW book currently published.

Talislanta 4 had everything in one neat package. Tons of inspiration, all the rules you need.
Conan D20 has all you need. The rest is just splat books and pastiche...

And I think you could run a D&D campaign with just the PHB. Could be interesting if you have to build all your monsters and magical items yourself. And keeping item construction out of the hand of the players is always a good idea. The wealth system of 3E is even worse than previous carts-of-gold fiascos.

D&D: The three core

WFRPG: The main book (I played a wizard and we don't have any of the others)

SR, Legend of the 5 Rings, Silhouette, Babylon 5, d20 Modern: Main book

Hackmaster: PHB, DMG, and some source for monsters (not necessarily the Hacklopedias, as from what I've hear an AD&D Monster Manual works fine).

Rolemaster 2e: Campaign/Charater Law, Spell Law, Arms Law. Companion I is a boon to any campaign, and I wouldn't run without it, but theoretically you could. You'd just have a worse initiaitve system and less fluid stats. Creatures & Treasures is also a great book to have, but not necessary unless you want preconstructed monsters instead of animals and NPCs.

Quote from: SosthenesTalislanta 4 had everything in one neat package. Tons of inspiration, all the rules you need.

Good example, Tal4 is marvellously complete.

That said, I do have a supplement that was never released with generic npc stats which is bloody handy.

@Jeff: The german Traveller Books I + II.
Number one was basically "The Traveller Book", but number two had all rule books and JTAS rules articles as well as Citizens of the Imperium and other cool stuff in it. They were made with a very sturdy cover, with some heavier fabric strengthening it. The binding was with yarn, and the cover illustration totally defines the coolness of traveller for me: