Keeping my iPhone charged is an important goal in my planning. It makes a reasonable ebook reader and it's relatively easy to keep charged with AAs, hand-crank power, and other means. It's easy to keep a bunch of entertainment and reference reading available.

The counter-argument is that paper books are more resilient to rough handling and don't need charging. I stuck the pocket version of the SAS Survival Guide in both my wife's and my BOB with that in mind, along with means for charging our iPhones.