The Screwtape Letters by C.S. Lewis
This is a work of fiction by a non-Catholic, but it is incredibly eye-opening, not to mention humorous. The story tells of a senior demon who sends letters with advice to his nephew on how to properly tempt his human “subject” and thus ensure he ends up in hell.
After you finish The Screwtape Letters, you might want to read Lewis’ science fiction trilogy: Out of the Silent Planet, Perelandra, and That Hideous Strength. Perelandra was especially fascinating to me because it essentially re-tells the story of Adam and Eve—on the planet Venus.