It really depends. There are two brands of "flavor": fluff and idea fodder.
Fluff is what you see in the slayers guides, much of the later Rokugan books, etc... laborious and exhaustive stories as if the author forgot that they were writing a role-playing game product and not a novel. Fluff delivers all too little value for the money AFAIAC.
Idea fodder is text like the descriptive text in a well written rulebook (say the class descriptive text in LE1, the spell descriptive text in R&R, and the plane descriptions in MotP: they deliver an idea with minimal overhead and don't try to deliver you a novella along with your book.