This could be something that you bought for a higher price than what most people would guess based on the item, or it could be something you bought for a normal price that has gained significant value as time has gone on.
What made me think of this question is a LEGO minifigure I got with my “The Hobbit: An Unexpected Journey” disc. It is Bilbo Baggins in a blue coat that was apparently only sold in that movie box only at Target stores. Even considering the exclusivity, I would have guessed maybe $10-20 for such a tiny piece of plastic, but there are sold listings on eBay from $80 to $225. I could possibly even get towards the higher end of that number since I still have everything in the original box in good condition. It’s not worth a ton compared to some other items people may own, but I think most people would not expect nearly that amount.
I have quite an extensive book collection between my wife and I who are both avid readers, turns out due to limited printings even unassuming non-“collector” books appreciate quite well. I have several books purchased for 20-30$ that are now worth hundreds if I wanted to part with them since the company just didn’t print many and doesn’t do reprints.
I switched to ebooks a long time ago, but had quite a collection prior to that, and was also the recipient of goodies from my mom’s massive science fiction book collection, including a first edition Dune (Chilton, 1965). My favorites are my signed Terry Pratchett and Douglas Adams books, though.
None of them are going anywhere.
I also have a decently sized book collection, but none that have gone up in value like that. Do you collect any specific types of books, or just whatever you think looks interesting?