As the saying goes, start at the beginning:
A Theory of Fun for Game Design by Raph Koster. Not huge on the mechanics of how to actually make a game, but it's a great starting point to begin to actually think about game design and not games production, games art, games programming, or whatever. They're all part of what goes into making a game, but they are not game design. If you only buy/read one book about game design, make it this one.
This isn't specifically on simulation game design, but it's always a great read for game design: