Backcountry first aid, medicine, and primary aid
This book covers it all. Do you need to know about that snake that bit you? Look in here? Did someone fall and break their ankle? This has you covered. Do you have rampant diarrhea? How dangerous is that rash?
This book will have you covered if you're a dirt bag weekend climber or a serious career-long member of search-and-rescue.
This book is over 2000 pages long, so you can't take it with you backpacking. But if you're serious or committed, this is the most authoritative source in the world.
I haven't studied it all, but before going to Joshua Tree I intend to study the Hell out of the chapter on snakes.
Climbing
If you want to climb hard, you have to train. This is THE book you need to read. It covers the mental, physical, and technical aspects of climbing. It gives a clear path to improving your climbing and defining your personal training needs.