I've been studying different formations where gold seems to occur, so many variables, I'm wondering if the the event under it all is the MOST important!
I'd recommend taking a look at Tom Bohmker's books. There are many technical articles and books on gold formation, but very few that discuss it in layman's language with the addition of field experience as Tom does. While his books are geographically-specific, they include a wealth of general knowledge as well. They are gems!
Another book to consider is Fists Full of Gold, by Chris Ralph.
One of the first prospecting books I read was one I downloaded for free. I liked it so much that I ordered a new paperback copy off of Amazon for about $8.
A Study of Ore Deposits for the Practical Miner - 1908 by JP Wallace