University libraries may be clustered around urban centers, but a lot of those are in places that might plausibly not be subjected to bombardment, such as South America. Also, university libraries are far more numerous and disperse than the factories which manufacture computer hardware. I also assume that preservation of books will be a priority for at least some people even in near-apocalyptic circumstances, and that material conditions would improve markedly after only a few years. Even the continuation of many governments seems likely to me. Also you can't eat books and they have little value to looters, so that should help.