I own a lot of books. My spreadsheet shows over 1100 books in my scholarly collection, and I suspect there are a few that have escaped the spreadsheet.
I don’t keep track of the fiction, as there’s a certain amount of flux in that collection. Some favorites have been with me for most of my life, while other books come in, get read, and then given away again.
Because I have so many books, I keep the scholarly collection ordered by Library of Congress call numbers. Roughly. Due to various moves, sometimes they’re only sorted by letter(s) and then maybe by period: DA is English history, and I can usually tell by the title whether they’re 13th, 14th, or 15th century. I have ambitions to get them all properly organized, so in my spreadsheet I try to give each its proper call number.
This is easy for books published in the US, which generally print an LC number on the publication page. It’s still easy for UK and European books that are owned by major US libraries that use the LC system, or which belong to some UK libraries that have adopted the LC system for convenience (IIRC, the Oxford History Faculty does this).
Then there are the truly obscure books, published by minor presses in the UK, owned only by repository UK libraries and various German libraries, none of which use LC numbers. This is where I get into the Library of Congress site and start downloading PDFs explaining their system, so that I can assign an appropriate number in my spreadsheet.
Why do I do this? (A) I need some sort of system, and (B) most of the libraries I use are on LC, so (C) it’s convenient to be able to look for books among their usual friends, no matter where I am.
I actually love getting into the LC’s PDFs and figuring out how to catalog my weird books. It’s so cool that the LC provides information about their system to anybody who wants it. I may have only one book in the CB category, but I know that is its proper area.