I only have around 350 books organized in total. This is roughly how I've gotten them laid out.
>Even though I've split Engineering and Health from Science, I still think I'm going to need to further subcategorize it into fields of study at some point.
>Religion, Philosophy & Politics as well will eventually need subcategories into pol natsoc, soviet leftypol, allahu ackbars, le happy judaism, christianity, and general assortment.
>"Engineering Unsorted" exists because I downloaded some books in a package and I didn't get around to finishing sifting through it all.
In terms of deciding what goes where I simply decide which field the book is primarily the most relevant to. Perhaps I should have a "tags" system, but this method is working for me for now.