From the Back of the Book:
Until now, there has been little reason for adventurers to give guilds a second thought. While often seen as places to find goods and services or hirelings, guilds can form the backdrop for many exciting adventures on many fantasy worlds where power and prominence in shaping nations, cities, and cultures depends on the guilds and money. Much excitement and intrigue can lurk among the shadows and rooms of the guildhalls, as those with secrets and coins quickly learn...
Guilds hold strengths and influences hardly available from dragons’ lairs or the darkest of dungeons. Guildcraft details the powers and privileges of guild membership, as well as new knowledge and details on the underpinnings of your d20 worlds. Adventurers of every class and skill have new opportunities for both enlightenment and profit.