For me it’s two things: the ‘used book smell’ and the treasures you can stumble upon-open, out of print/hard to find books. Case in point: a few years ago I bought a first edition novel called ‘Lord of the World’ by Robert Hugh Benson, published in 1908. I paid about $106 with tax. It’s in pretty good condition for its age (minus the dust jacket) with some fading on the cover’s lettering. I’ve seen 1945 editions selling for over $400. However, I recently seen the same edition as mine on ABE Books going for $5000. I’m fairly certain that mine is worth we’ll above what I paid for it even if it isn’t as much as the one on ABE’s website.