Build Worlds, Second Edition by Dancing Light Press for a while. It retails for $4.99 and tops out at just shy of 90 pages at 89. The cover art is excellent and the table of contents is tight. The book is divided into three main headings, Intro, Format and Elements, each one bigger than the last.
From the links above, you can see I am heavily invested in World Building for my B/X and AD&D campaigns. Back in the day, I picked a wonderful book for second edition AD&D called Campaign Sourcebook and Catacomb Guide. While I had a low awareness of second edition and a slight dislike of it, this book was a revelation to me. Now, it isn't exactly built for world building, but it drags you through the process of designing the adventures within your world.
Building Worlds is specifically meant for world building and is totally rule set agnostic. That means it should rejuvenate your eyes, giving the reader fresh insight in to their current campaign or a new one they are planning.
Go ahead and give them both a try. Both are available at DriveThruRPG from the links above.