"Formal" library software (e.g. Evergreen) may be the answer for repo
wide searchability of meta data, but when others share interest, they
may only be able to store, and or may be only particularly interested
in a sub-category or two such as health and agriculture/gardening, so
it might be best anyway to use a handful of top level "general
categories" to reduce our maximums down from 50K books per dir, to 10
fold fewer at least.