For me, the best fluff comes from non-game related material. The book Marco Polo: From Venice to Xanadu is chock full of great stories, most fanciful, that he recorded during his travels. Michio Kaku's Physics of the Impossible has a ton of useful ideas for space adventures, such as baby universes, or time machines that allow you to only go forward or back to when the machines were built. File off the serial numbers and you can use these in fantasy campaigns.
The Balkans by Dennis Hupchick is a history of the Balkans and includes detailed information on how the Ottomans organized their empire, including the recruitment of non-Muslims as the sultan's slaves, called devsirme. To modern eyes this seems barbarous, but in practice it offered significant opportunities for social mobility that were otherwise denied to the sultan's Christian subjects. Some parents would bribe officials to select their sons - even some Muslims tried to get in on the action. This was because such slaves were given opportunities based on individual merit and performance, the best rising to the highest levels of government.
You just can't make up stuff like this.