Depressing lack of books on animal husbandry, crop management, basic mechanics and machining, and math/physics/chemistry text books and field manuals.

Philosophy is good and important, but much easier to carry verbally (out re-invent) than the thousands of years of trial and error of "what's this red stuff in the dirt and what can I do with it?"

Then again, assuming that we'll lose civilization but terrain the ability to read written English is somewhat hard to justify, too...