Waiting For God - Simone Weil - Letters to a French priest from a Jewish woman who was "...taken by Jesus..."
Finite & Infinite Games - James Carse - “There are at least two kinds of games,” states James P. Carse as he begins this extraordinary book. “One could be called finite; the other infinite.”
Does God Exist? - Keinosuke Johan Miyanaga - Many great philosophers have answered this question. Here is an answer that is complete and challenging.
Help My Unbelief - William O'Malley, SJ. - Father O'Malley played the senior priest in The Exorcist. He writes entertainingly and convincingly about doubt.
Mere Christianity - C.S. Lewis - A series of radio lectures Mr. Lewis gave in the 1940's in England. A famous book that puts forth what Christians believe.
The Screwtape Letters - by C. S. Lewis - audio read by John Cleese - Others have narrated this entertaining book, but Cleese is the one to get if you can find it. A senior Demon gives advice to an apprentice on how to tempt humans to the dark side.
God Is Not One - Stephen Prothero - An overview of the world's major religions.
And a few words from the other side...
God Is Not Great - Christopher Hitchens - To truly appreciate Hitchens' arguments and skill as a debater, go to Youtube and search on his name. Yes he was a polarizing individual. But never boring.
The End of Faith - Sam Harris - More smart guy taking on faith. Again, Youtube is the best place to get the full impact.
The God Delusion - Richard Dawkins - See above.
And for the pure secularist...
The Fabric of the Cosmos - Brian Greene - A physicist with a knack for explaining the near unexplainable.
A Brief History Of Time - Stephen Hawking - This book has outsold the Bible, in dozens of languages. We all want to know where we came from and where we are going.