Core Books for Campaigning
There are pretty much unlimited books and manuals for different RPG games and gaming. I am listing the top four games (as gathered from cross referencing different, unofficial lists) for their gaming books and manuals plus one non-manual book.
Dungeons and Dragons
As per the official website. The core products for Dungeons and Dragons (4th edition) is as follows:
Wizards RPG Team. Dungeon Master's Guide. 4th. Seattle: Wizards of the Coast, 2009. Print.
Wizards RPG Team. The Player’s Handbook. 4th. Seattle: Wizards of the Coast, 2009. Print.
Wizards RPG Team. Moster Manual. 4th. Seattle: Wizards of the Coast, 2009. Print.
These three books, together, create a core playing experience for Dungeons and Dragons. There are several dozens of supplemental books with additional monsters, races, classes, worlds, etc. These three books, however, are the only three essential pieces in creating, running, and participating in a Dungeons and Dragons campaign.
Castles and Crusades
Chenault, Davis. Castles and Crusades Complete. 5th. Chicago: Troll Lord Games, 2012. Print.
This is a conglomeration of the four core materials for running and participating in a Castle and Crusades campaign. It includes The Players Handbook, Monsters and Treasure, Of Gods and Monsters, and The Castle Keeper’s Guide. This is a much simpler gaming system and has few other supplemental texts. This book could be considered a complete collection of everything needed to play Castles and Crusades.
Pathfinder
Bulmahn, Jason. Pathfinder Roleplaying Game: Core Rulebook. Chicago: Paizo Publishing, LLC., 2009. Print.
Bulmahn, Jason. Pathfinder Roleplaying Game: Advanced Player’s Handbook. Chicago: Paizo Publishing, LLC., 2009. Print.
Bulmahn, Jason. Pathfinder Roleplaying Game: The Pathfinder Bestiary. Chicago: Paizo Publishing, LLC., 2009. Print.
These three books are the tools needed for running and participating in a Pathfinder campaign. There is not, like the previous two games, a game master handbook. There are supplemental products that may acts as further tools for the game master, but these are not needed. This game is a bit more complex than Castles and Crusades, but not as vast as Dungeons and Dragons. There are other products for this game, but this collection would be more than enough to run a campaign.
Mutants and Masterminds
Kenson, Steven. Mutants and Masterminds: Hero's Handbook. 3rd. Chicago: Green Ronin Publishing, 2011. Print.
Kenson, Steven. Mutants and Masterminds: Gamemaster’s Guide. 3rd. Chicago: Green Ronin Publishing, 2011. Print.
Kenson, Steven. Mutants and Masterminds: Threat Report. 3rd. Chicago: Green Ronin Publishing, 2011. Print.
In taking a step away from the high fantasy genre, Mutants and Masterminds is a highly successful RPG centered on a comic book, superhero universe. These three books equal out to the previous player’s handbook, gamemaster guide, and monster manual of the other collections. This game offers many downloadable suplements (all these books are offered as PDFs). Once the campaign has begun, one might find it necessary to purchase these additional Threat Report downloads for new adventures.