for me, the books I use and have used the most when designing a new campaign are the following:

Gygazian Fantasy Worlds series from Troll Lord Games

Book 1:  the canting crew:  tells you how to create a realistic underworld, with various criminal sorts.  Essential for good gritty city play
Book 2:  World Builder:  A veritible cornucopia of lists, tables, charts and definitions on all things fantasy world related,  split into 3 sections, section 1 covers "Stock-in-trade" items such as armour, weapons, a mix or fantasy items for sale in a bazaar, taverns and transportation.  section 2 covers geography such as geographic features when designing a world, flora and fauna.  section 3 covers dwellings,  construction types, building types, furnishings, typical rooms in different dwellings and poulace details.  Essential reading IMO for anyone building realistic settings

Book 3:  Living Fantasy:  This is the setting designers dream.  Where book 2 covered the macro level, this book covers the micro level.  Urban and rural landscapes,  water transportation and how cities were built near easy egress, civilized communities, the underground domain.  Culture, temporal considerations and a great overview of "a day in the life" of many different city archetypes (bailiff, beggar, mage, shopkeeper, wealthy merchant etc)

Book 4:  book of extrodinary names:  this book simply rocks, and now never leaves my side.  it has 100,000 different names in this book.  if you want to name something on the fly, this book is for you.  you want to create a germanic feel for a region, flip to german names and there you go.  you want random name genorators, here you go.  simply essential :)

Book 5:  Insidiae:  designed for fantasy writers and campaign builders, this book details how to build a story arc,  and gives numerous examples on how to build a meta story for your world (or region, or time in history)...

Book 6:  Nation Builder:  So,  now you have a world, a geography,  the common cities and towns, a naming convention and story arcs to fill years of gaming, well, this book helps you detail the nations that populate your world.  you want an empire, a city state, a kingdom, well, this book will help you build it in a consistent manner.  it goes into detail about government types, community types, and cultural characteristics which bind a nation together.

Book 7:  Cosmos builder:  so now,  the world is complete, it's time to establish mythos and mythology.  Cosmos builder teaches you how to create a celestial and plannar realms and how to populate it with gods, demons and various otherworldly types.



So far and combined, these represent over 1000 pages of world and campaign building advise from the man who invented the whole RPG thing to begin with.  while not written by him, he directed the creative process throughout.  for me, these are the most valuable books that I own when helping design and create worlds for my players to frolick around in :)  what resources do you use to make your games better?

Cheers,
J.