The Problem of Pain was published in 1940. Lewis wrote The Problem of Pain during the first autumn and winter of WWII. The book was read chapter by chapter to "The Inklings," the literary group he met with weekly, which included Charles Williams and J.R.R. Tolkien. Lewis dedicated the book to the Inklings.
Rev. James Welch, Director of Religious Broadcasting at the BBC, read The Problem of Pain and was impressed such that he approached Lewis about doing a radio series that eventually became Mere Christianity.
In The Problem of Pain, Lewis first does a "ground-clearing operation" and lays a new foundation before building his main argument. Consequently, he doesn't actually get to the purpose of suffering until chapter 6. (Shepherd's Notes Series, C. S. Lewis's The Problem of Pain/A Grief Observed, Broadman & Holman, 1999, p. 10.)
Alcorn, Randy, Heaven (2004)-Alcorn, a fan of C. S. Lewis, combs all of Scripture gathering clues about the nature of heaven.
Alcorn, Randy, If God is Good: Faith in the midst of suffering and evil (2009)-addresses the perennial question of why a good God allows suffering.
Guinness, Os, Unspeakable: Facing up to evil in an age of genocide and terror (2005)-Guinness addresses the suffering caused by other humans and how we are to find hope in the face of evil.
Lewis, C. S., The Great Divorce (1946)-Lewis supposes that a busload of residents take a day trip to heaven. When offered the chance to stay, who will seize the opportunity?
Martindale, Wayne, Beyond the Shadowlands: C. S. Lewis on heaven and hell (2005)-Martindale traces the themes of heaven and hell as found in Lewis's works.
Shepherd's Notes, C. S. Lewis The Problem of Pain & A Grief Observed (1999)-chapter-by-chapter summaries, notes, and questions for two of Lewis's works. Similar to Cliff's notes.