For physical books, I keep a google doc inventory. Its not currently complete as I haven't had time to sit and inventory all my shelves & the bins I need to unpack, and some on the list I need to mark for deshelving. There are currently 267 on the list I think, but I have somewhere in the ball park of 1000 physical books. My focus leans more to witchcraft, folk magic, folklore, herbalism and peripheral topics. (Note that owning a book does not mean I place any sort of value, belief or agreement with its contents/author. I'll read just about anything, but some books I have only because it came bundled with others I was interested in.)
I think a few of my favorite books in my collection are probably:
1856 English copy of The Long Lost Friend - John George Hohman
1860 The Night Side of Nature - Crowe
1899 The Magic of the Horse Shoe - Lawrence
1864 Saxon Leechdoms vol 1-3 (Leechdoms, wortcunning and starcraft of early England)
I've been meaning to organize and inventory my digital library for quicker reference, but... The mere thought of that is horrifying really. I've saved so many books, articles and such over the years that its going to be its own mountain of a task.