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Table of Contents:
"Answers to Questions on Christianity" (1), (2), (3)
"Before We Can Communicate" (3), (4)
"Behind the Scenes" (3), (5)
"Bulverism: or The Foundation of Twentieth Century Thought" (3), (4), (6)
"Christian Apologetics" (1), (2), (3), (6)
"Cross-Examination" / "I was Decided Upon" & "Heaven, Earth and Outer Space" (1), (3), (5)
"Dangers of National Repentance" (1), (3), (5)
"Delinquents in the Snow" (3), (5)
"Dogma and the Universe" / "Dogma and the Universe" & "Dogma and Science" (1), (3)
"Evil and God" (3), (5)
"First and Second Things" (3), (4), (6)
"God in the Dock" / "Difficulties in Presenting the Christian Faith to Unbelievers" (1), (3)
"'Horrid Red Things'" (1), (3), (4), (6)
"Is Progress Possible?"
"Is Theism Important?" (2), (3), (6)
"Man or Rabbit?" (1), (3)
"Meditation in a Toolshed" (3), (4), (6)
"Meditation on the Third Commandment" (3), (5)
"Miracles" (1), (3)
"Miserable Offenders: An Interpretation of Prayer Book Language" (3), (5)
"Modern Translations of the Bible" (1), (3), (4), (6)
"Must Our Image of God Go?" (3)
"Myth Became Fact" (1), (3)
"On the Reading of Old Books" (1), (3), (4)
"On the Transmission of Christianity" (3), (4)
"Priestesses in the Church" (3)
"Rejoinder to Dr. Pittenger" (2)
"Religion and Science" (1), (3)
"Religion Without Dogma?" / "A Christian Reply to Professor Price" (1), (2), (3), (6)
"Revival or Decay?" (3), (4)
"Scraps" (1), (3), (5)
"Some Thoughts" (1), (3), (4), (6)
"The Decline of Religion" (1), (3), (4), (6)
"The Founding of the Oxford Socratic Club" (2), (3)
"The Grand Miracle" (1), (3)
"The Humanitarian Theory of Punishment" (3), (4), (6)
"The Laws of Nature" (1), (3)
"The Pains of Animals" (2), (3)
"The Sermon and the Lunch" (1), (3), (4)
"'The Trouble With "X" ...'" (1), (3)
"Two Lectures" (3), (4)
"Two Ways With the Self" (1), (3), (5)
"Vivisection" (1), (3), (4), (6)
"We Have No 'Right to Happiness'" (3)
"What are We to Make of Jesus Christ?" (1), (3)
"What Christmas Means to Me" (1), (3), (5)
"Work and Prayer" (1), (3)
"XMas and Christmas: A Lost Chapter from Herodotus" (3), (4), (6)
Notes:
(1) also published in "The Grand Miracle, and Other Selected Essays on Theology and Ethics"
(2) also published in "Timeless at Heart: Essays on Theology"
(3) also published in "Essay Collection and Other Short Pieces"
(4) also published in "First and Second Things: Essays on Theology and Ethics"
(5) also published in "Christian Reunion and Other Essays"
(6) also published in "Compelling Reason"
Recommendations:
If you are interested in Lewis's shorter works, my best advice is to get "Essay Collection and Other Short Pieces", which, as of the time of this writing, is available from Amazon UK but not Amazon US. That collection consists of about 130 short works by Lewis. The works in that collection are mostly, but not exclusively, Christian.
If your interest in Lewis's shorter works is restricted to those on Christianity, and your budget or enthusiasm does not run to "Essay Collection and Other Short Pieces", then my second-best advice is to get any or all of the following (they don't overlap significantly, and between them they include most of Lewis's shorter Christian writings):
"God in the Dock - Essays on Theology and Ethics"
"The World's Last Night and Other Essays"
"Christian Reflections"
"The Weight of Glory and Other Addresses"