I've never bought a Frank Sheed book, although I see them in the catalogs. I'll have to try the used book sites, after searching the library catalog.
Flannery O'Connor doesn't reach me, for some reason. What I've read leaves me saying, "Why did I put myself through this?"
I highly recommend Frank. He writes beautiful prose and it is so easy to comprehend.
I think one must read the letters of O'Connor before reading her work. She was dying, knew she was dying and that had a huge impact on how she needed to portray God's grace. That is why I call it the "sledge hammer" approach.