Michael Frye, Photographer's Guide to Yosemite
Michael Frye is well-known as one of the best nature photographers working today and he has devoted much of his life to photographing Yosemite. This is a relatively comprehensive book in that Frye gives tips on equipment and technique, best places to go, famous spots, tiems of day and times of year to get specific photos and so forth. It's a good book for anyone who is serious about photography, but indispensable if the primary purpose of your trip is photography. It's well worth trying to get this book well in advance of your trip so that you don't come expecting to take pictures of phenomena that are purely seasonal.
There are a couple of other books in this category that get good reviews on Amazon, but I don't know them personally. If any of you own one or both, take a moment to say why you like and why you would recommend it (or not, actually, as the case may be). If you think one of these is better than the Frye book, please try to be specific. The idea is to help people make a decision, not just voice an opinion.