I own a lot of books. Nowadays, I mostly buy Kindle books, since the bookshelves had to go when we got twins. I now own about 700 books on Kindle, of which 400+ are read at least once. This is the result of one single Twitter account (mostly), @SFSignal, that tweets sales of SF and other fantastic fiction, under the tag sff_eBookDeal. Some of the books go for as low as $1-$2, and that’s where the majority of my digital library comes from.
I also own a lot of actual, physical SFF books. Nearly all paperbacks. We used to have a high shelf for my books along the walls of our entry hall, but when we switched apartments a few years back, that had to go. It swallowed at least 150 books, that now lying in boxes in the cellar storage.
When we started the Matmolekyler project, we started out with buying useful books. I have a bunch of those in my bookshelves (including some I got myself, or as gifts: several books by HaroldMcGee, two priced *signed* books by Gordon Shepherd). Since then, I try to buy food-science-related books when I find them at reaonable prices. That collection is now about 50 books.
I also went through a period where I was thinking a lot about food and cooking history, so I tried to find some of the earlier real cookbooks (and school household training books) from the early 1900s. There are typically cheap and abundant at used book stores, so by now I have quite a few. Including one I got at a museum, which is a copy of a 1800s cookbook along with a companion book written by a historian). And a set of two old ~1800s household encyclopedias in a new edition from a Swedish agricultural society (I had to order them special and pick them up in person). Let’s say, another 40 or so books.
For 2021, I hope to rearrange all of our (mine, really) bookshelves so that I have more SFF books accessible (since I want to re-read them anyway). I hope to inventory and index all books, and maybe even get rid of some. I need to find some good book inventory software – I have Goodreads but haven’t really used it for sorting books yet.