I kept the important stuff in dead tree format, if the lights go out. Things like The Way Things Work and other technical and how to guides. I should also have a copy of Henleys Formulas running around, but I couldn't find it the last time I looked. Important to have technical guides, for those lights go out scenarios.I've got more stuff in digital form than not, but not everything I have is digital. And if the lights go out for any length of time ove a day, I may need the dead-tree copies.