This authoritative volume, now in its 4th edition, is the one I turn to the most for answers when I have questions about backcountry medical issues. This book provides answers to this and myriad other issues. On a historical note, I acquired the 2nd edition of Medicine For Mountaineering shortly after it was published in 1975. Comparing the 2nd and 4th editions provides an interesting window into how backcountry medicine has advanced. For example, understanding of hypo- and hyperthermia have expanded greatly, with corresponding advances in treatment protocols.
Water treatment is another area that has expanded considerably.
This book is too bulky to carry in your backpack, but I consider it a must have element of my home library. Their Causes and Avoidance by Stephen Herrero. A classic treatment of the scientific evidence illuminating the factors that precipitate or covary with bear attacks on humans. The Search for Mallory and Irvine by Peter Firstbrook. The discovery of George Mallory's long-missing body on Mt. Everest generated a plethora of grotesque magazine covers and a flurry of books.