I bought three books as I prepared for my first race (a sprint, which my team won thanks to the good advice found in all three of the guides discussed below): Don Mann and Kara Schaad's Complete Guide to Adventure Racing, Ian Adamson's Runner's World Guide to Adventure Racing, and Barry Siff and and Liz Caldwell's Adventure Racing: The Ultimate Guide. Of these, I found the Complete Guide good to start with, Adamson's book a standard guide to go back to again and again, and Siff and Caldwell's an ongoing delight.
The layout of Mann and Schaad's work is good for those just starting--lots of sidebars, whitespace, and basics. Adamson has professional prestige and his book is filled with the opinions/wisdom of someone with hardwon wisdom. I sensed though that as a high profile professional adventure racer, he is obligated to sponsors. His gear recommendations, which I gobbled up at first, began to seem more like celebrity endorsements by the end and I began to wish there was a more objective equipment guide. I have no doubt that Adamson's equipment recommendations are excellent; I would have liked to have gotten a better sense of comparison, though, to help me better understand why a Paladin harness was the recommendation rather than an Alpine Bod-- ie. what are the little things I am really looking for when I go out and buy gear for a discipline I don't yet do?
On Siff and Caldwell, I think the text portrays fingertip wisdom as good as Adamson's and the color photo section is great. Having read all three a couple of times through, I find myself picking up Siff and Caldwell and browsing it more than the others.
Mann and Schaad are on the bottom of the pile. I think that has something to do with the layout--which hints of Idiot's guides. I've certainly not worn out their advise but the layout has gone stale.
Recommendations: If you are completely new to the sport, and to more than one discipline, get the Complete Guide and read it first. Once you are hooked, you have to have the other two.
One area that these books don't purport to cover well is land navigation. A great book for learning those skills is Burns, et. al. Wilderness Navigation. I have taught from it several times in college level classes and it is a better fit for adventure racing than for what I have been using it (fundamental map reading and compass navigation).