Currently I have a game world and a rule system that I created to run the world. I have came into problems of how to introduce my game world in the core rules. In some games I have seen the world introduced in the beginning others are so intertwined that they couldn't be written apart. Introducing the world in the beginning would seen like a story to me and neither the system or the world are intertwined.

Does anyone have any ideas on how to put the rules and the game world seamlessly together in one book.

Well, if they have nothing to do with each other, it's gonna be hard to make them "seamlessly" fit, I would imagine.

I would present the coolest thing about the game first off. If the cool thing about your game is the hot dice mechanic, talk about it. If it's the flying purple monkeys with lazers on their heads, talk about it. Catch the interest of the dude that skims introductions in the book store.

Then I would put the discussion of the cool thing first, and the other stuff later.

Thats how I would do it, YMMV. A pretty standard format is to divide your (physical) book into "Section/Chapter/Part 1" with the mechanics and a Part 2 with the setting, or sometimes reversed.

Hope that helps.