Hill Fit. For those that don't know, Chris Highcock runs the Conditioning Research blog - a hugely respected and popular fitness site pertaining to health, performance and fitness in general.
Chris has identified an obvious but much neglected niche in the armoury of the dedicated hillwalker and rambler; strength. While most outdoor enthusiasts will pay attention to their primary hardware (gore-tex shell, fleece, footwear, rucksack, GPS etc....), few would consider strength to be important.
In fact, short of running around the block a few times a week to improve 'cardio', few walkers I know would actually engage in serious strength work. This is a mistake - something Chris makes a compelling case for in Hill Fit.
Chris takes you through the research and helps you put together a simple, short but effective routine that relies primarily on bodyweight and low-tech equipment (something which gets the Natural Messiah thumbs up!). Easy and advanced variations are given for each exercise which you can work through, ensuring intensity is maintained as your strength progresses.
The book is 'focused on something that every walker, hiker and backpacker needs' and certainly, if you've never considered training for strength before heading to the hills, I'd recommend you get your hands on a copy.