I learned this from a bibliophile minister many years ago: Any book worth reading is worth keeping. Any book worth keeping is worth having in hardback, if at all possible. Buy hardbacks (used if possible) and keep them. Bookcases aren't really that expensive. (Toss the occasional purchases not worth keeping, and don't worry about them.)
Oh, and do whatever it takes to keep mold out of your library. Moldy books that you decide to keep, not replace, belong only in sealed plastic boxes treated with borax, sodium carbonate, and/or chlorine dioxide, and preferably stored outside the house!)
Building a library is like planting trees - you don't do it so much for yourself as you do for those who will come after you. I have a digital library, too, but I don't expect it to be usable at all in a century...
Oh, and do whatever it takes to keep mold out of your library. Moldy books that you decide to keep, not replace, belong only in sealed plastic boxes treated with borax, sodium carbonate, and/or chlorine dioxide, and preferably stored outside the house!)
Building a library is like planting trees - you don't do it so much for yourself as you do for those who will come after you. I have a digital library, too, but I don't expect it to be usable at all in a century...