I've picked up some very cheap strategy guides for computer games recently, namely 'Warhammer Online: Age of Reckoning' and 'Dragon Age II', and even though I've played both games I never bought them for the help they'd give me.
What I got in the books were detailed maps, characters, weapon ideas, adventure and quest ideas, illustrations and a whole slew of new ideas. The books are like system-generic sourcebooks and, even though I won't be using them in the settings they were intended for, there are plenty of bits and bobs in there I can use for my current games.
Once games have been out for a long time and the strategy guides become pretty useless you can find them in all kinds of places - charity shops, second-hand bookshops, discount bookstores and the like. They might not be an accurate fit for your game but there's bound to be something in there that you can cannibalise. I think I'll be seeking out more cheap books like this in the future to help flesh out my home-made roleplaying campaign settings.