Hm, kind of both... Mostly, yeah, I just have this massive file in my head of everything I've read. But I do also keep spreadsheets at work – literal spreadsheets – for a few high-interest areas.
For example, books with LGBTQ+ themes, because that's both an area that kids ask for specifically, and an area where we've really built up our collection over the last year, so I was involved in a lot of the book-buying decisions and know at least a bit about most of them, even if I haven't personally read them yet. I also have a spreadsheet of all our horror books, because that's a frequently requested area but conversely one I'm not well versed in at all, because I just can't stand to read horror, at all. So at least I have a list to refer to! I also made a spreadsheet of all our historical fiction, broken down by era, but unfortunately no one's ever asked for that. ;-)
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For example, books with LGBTQ+ themes, because that's both an area that kids ask for specifically, and an area where we've really built up our collection over the last year, so I was involved in a lot of the book-buying decisions and know at least a bit about most of them, even if I haven't personally read them yet. I also have a spreadsheet of all our horror books, because that's a frequently requested area but conversely one I'm not well versed in at all, because I just can't stand to read horror, at all. So at least I have a list to refer to! I also made a spreadsheet of all our historical fiction, broken down by era, but unfortunately no one's ever asked for that. ;-)