I'm a big fan of paperback originals that were published in the 50s-70s. These are books that were first published in paperback at a time when more respectable books were always first published in hardcover. Most of them are genre (sci fi, detective, spy, romance, crime, horror, action) and prioritize entertainment over literary appeal. They were the sort of books that people would buy near the cash register and read at the bus stop or during their lunch break.

My local non-used book stores barely stock any of the mass market format books that spawned the paperback original; everything has moved to the more expensive trade format. Fortunately there are decades of books still available in the old mass market format. You can certainly find them online and at used bookstores, but I recommend checking out your local library annual sale (https://booksalefinder.com/) for a fun and cheap way to acquire reading entertainment.