I really can't think of another book that is both religious and popular, and deserves its prominence in both categories.
There are theological works, and lightly disguised works on theology (Dante comes to mind), but they can't compete. Well, "Robinson Crusoe" by Daniel DeFoe was perhaps equally popular as I was growing up. Everyone I knew had both books, and had the stories of both down pat.
I recommend taking the time to read them.