This is THE classic prospecting text. My own battered volume is over 25 years old and I would never part with it. It must be good, since I continued to pack mine on my back through the mountains long after I had discarded most of everything else to save weight.
Professor Pearl covers everything that you need to know about both prospecting and working a small placer claim. Not only are the basics of prospecting (placer and hardrock) covered, but equipment, provisions, basic geology, simple tests, staking and filing, history, etc. I suppose some of the material is a little dated now (like the mining laws), but he tells you where to go for more information on filing a claim and such.
There is only one tip that I wish that he would have emphacized more, namely, when seriously prospecting for placer gold you have to get down to the bedrock. And not just what is currently under the stream either- dig down through the sediments on both sides of the flood plain. Also, there can be ancient deposits up on those higher terraces too- but you've got to dig, dig, dig down to the old bedrock. The gold collects in cracks in the stream eroded surface. It is a lot of work, unless you can get away with hydraulically washing off the overburden (which is completely illegal in the States.)