I like to collect rocks, but I also collect books. At a certain point, I learned to combine the two hobbies handily, and began to collect books about rocks.
A well-stocked bookcase is a must for the serious rockhound. From general geology books to identification guides to location field guides, a good book collection will expand your understanding of where to find your favorite rocks, and why they're there.
Here's a selection of a few of my favorite books for rockhounds.
A Field Guide to Rocks and Minerals (Peterson Field Guides)
A Field Guide to Rocks and Minerals is one of my favorite rock-related books, mostly because it includes tests you can perform to confirm the identity of your rock or mineral. It is arranged by mineral class, which can make it more difficult for the ameteur to find the mineral they are looking for, but it has a wealth of useful information - I consult mine regularly.
National Audubon Society Field Guide to North American Fossils
North American Fossils is a wonderful resource. Although it does not have everything [or it would probably be too heavy to carry on your collecting trips] it focuses on the most common fossils in North America. The photographs show what the fossil looks like "in the wild" rather than all cleaned up and polished, as some books do, making it much more useful for actual identification purposes.
Gem Trails of Northern California
This is an excellent collecting guide, with maps to many different sites, and detailed summaries of what you can expect to find there. I have a look at mine before every trip I go on, just to see if there's any interesting sites nearby I should visit while I'm in the area.
Penguin Dictionary of Geology (Penguin Reference Books)
Ever run into some obscure geology term, where you just couldn't figure out what the heck it meant? This is the book for you. It's packed full of geological definitions, and you'll never have to wonder about another one. Plus, you can impress all the folks at the rock club by using words like "endometamorphism" in casual conversation.
Roadside Geology of Northern and Central California
This book is not aimed at collecting, but if you have an interest in geology, you'll love it. You'll learn about the geologic features you might drive past every day, and how they formed. The downside? You'll have to remember to pay attention to where the road is, and not stare off at the road cuts too much while you're driving.
Then again, you're a rockhound. You were already looking at the road cuts.