When I go to yard sales, used book stores, flea markets, or antique stores, I tend to gravitate towards these kinds of books, and, if a volume has an appeal of some sort as to content, and the price is right (I'm still frugal as the day is long), I might very well bring it home with me.
For a while I felt kind of bad about this and wondered if I was being frivolous. But what I have found, as I read these books, is that often they turn out to be real treasures! I've read some old books that have been very entertaining, well-written, and unusual in a variety of ways, and have often closed one of these with a sigh of satisfaction and placed it back on the shelf with a feeling of well-being. So I've concluded that this "hobby" is enriching my life and, as long as my frugality stays firmly in place, doesn't need to induce guilt feelings. For now, anyway.
My most recently read "old book": The Royal Road to Romance by Richard Halliburton (that's "romance" in the old sense of the word - meaning adventure, pageantry, heroic deeds). My current "old book": The Good Earth by Pearl S. Buck.