The SAS Survival Handbook by John "Lofty" Wiseman is hands down the best wilderness survival book that I've ever read. I've had several copies over the years, and it's worth every penny.
Wilderness survival has been a hobby of mine for years, and I've read more survival manuals than I can count. The SAS handbook is just stellar, and if I had to pick just one, that'd be it.
Plus it comes in ebook format. I haven't been able to justify buying this in ebook yet though. I still have three paper copies of various editions.