Navy Electricity and Electronics Training Series, and a wealth of other stuff mostly in PDF: http://www.hnsa.org/doc/index.htm#neets
Like a wise man once told me, knowing is half the battle :)
Lots of military medical manuals.
http://www.cs.amedd.army.mil/borden/
Hesperian.org publishes medical books for distribution to impoverished people. I've got copies of their Where There is No Doctor, WTN Dentist, and A Book for Midwives. Don't laugh at that last one. Having home-birthed our last two children, with the help of midwives, I figure I may be the most/only experienced person around in a really bad situation. (I get shudders thinking about it. Things would have to be pretty bad if you need me to midwife)
The Little Herb Encyclopedia
http://www.woodlandpublishing.com/woodland-bestsellers/little-herb-encyclopedia-third-edition.html
This is a good book that my wife uses, it covers what herbs for each different illness and how to mix it. We have used this book to take care of illnesses with success. If the time comes when we don't have the ability to see a Doctor we will have to take care of ourselves.
Joe
High-Efficiency Antennas for Hand-Held Radios:
http://www.ked-wireless.com/documents/High%20Efficiency%20Antennas%20for%20HT%27s%20-%20QEX.pdf (http://www.ked-wireless.com/documents/High%20Efficiency%20Antennas%20for%20HT%27s%20-%20QEX.pdf)
Gil.
Lots of stuff over the years on the shelves, or printed & bound.
I tend to think of "must have" stuff as those things that are, in overall value, those I would bother to head out with, and which certainly don't encumber more than a field-jacket pocket. The old DoD antenna handbook (vs. the venerable & valuable ARRL handbook) is an example. Both are great; one is portable, one less so. A field guide to edible plants w/quality pics might be another.
One of the best "outdoor guide books" is a 1970'S Boy Scout handbook. Simple basic information and a few drawings.
73'S KA5SIW