The two strangest ones I can think of were fairly recent.

One was in 2019 when I was working at a local museum; one of my tasks was to sit in the ticket booth, which was generally pretty quiet and afforded the opportunity to listen to the radio or read when no one was around. One day I came to work and had forgotten my book, but was happy to find that an anonymous coworker had left a battered old paperback in the break room. I borrowed it for my shift and then returned it, but no one ever came back to claim it and no other coworker I asked knew who it belonged to, so eventually I just took it home. As it happened, it was The Way of Kings (which I had never heard of) and I really enjoyed it, but you can imagine my surprise when all of my friends started reading it and raving about it in 2022 ¯\_(ツ)_/¯

The second story was when I was living and working in France in the summer of that same year. I had brought a few English books with me, but quickly worked my way through them and was down to borrowing books from my friends or searching for English-language books in the local stores. The residence where we were staying had a number of mail lockers downstairs, which our group didn't use but which we had to pass by in order to leave the building. One day I was on my way out when I happened to glance at the row of lockers and noticed a book sitting on the top - it seemed like it had been left on the floor and some passerby had stuck it up there to keep it from being stepped on. I pulled it down out of curiosity and realized it was an English-language copy of Good Omens, my favourite book of all time - and it was the edition I owned, the 90s edition with the black cover. The serendipity of finding an English-language copy of my favourite book in my favourite edition in the locker room of a university residence in small-town France was too good to pass up, so I took it with me and it kept me company for the rest of my travels!