Thinking about RPG sourcebooks as antidotes to the blank page.

If I want to invent a town for an adventure or a story, and am unsure how to characterise it, having something like the Pathfinder Gamemastery Guide describing kinds of attributes a settlement might have is useful - it turns that blank page into some brainstorming categories.

On the other hand, if I have a strong idea already, I may not find such an aid useful, or only to round out some forgotten details. But we'd also want to be wary of the alternate trap - letting the design aid constrain ideas only into forms which fit in its boxes.