The Kobold Guide to Game Design is an excellent resource for any game designer or GM. It offers over 100 pages of essays by 11 industry-leading professionals such as Wolfgang Baur, Monte Cook, Chris Pramas, and others.
Topics cover a wide range of worldbuilding advice, from basic overviews like "what is worldbuilding?" to more in-depth analysis about different kinds of worldbuilding, how to build a "world bible," considerations when developing a pantheon of deities, mapmaking, and more.
As someone currently working on a new world, I found this book immensely helpful. It helped me think of my world in new ways and consider things I hadn't before. I came away with several pages of additional notes that I can now put to good use; I've considered the setting's geography, technology/magic level (and how the two interact), the nature of the gods, how guilds and secret societies interact, etc.
I can see this book as being a valuable reference -- something to come back to time and again as you continue to flesh out a setting. You may read it cover-to-cover like I did, but as you start to focus on one area of your new world you may find yourself coming back to individual essays for inspiration.
The book is simply laid out and logically designed. In general, each of the essays flows logically; "What is worldbuilding?" comes first to explain what the book means by the phrase, and the end of the book contains information about building on a world that is already part of a licensed property (this essay contains some really unique industry information about dealing with licensers). Three essays about religion are all grouped together, and everything else just seems to fit into place.
As a primarily text-based book, the layout is simple. There is little art, but the book doesn't really need any. The essays are really what count, and they all hit home in their own way.
Definitely check out this book if you're a game designer or GM who likes to build homebrew settings (or even build on existing settings). It's a top-notch book from start to finish.
[5 of 5 Stars!]