1. Mere Christianity by C.S. Lewis
In this, his finest book I believe, Lewis strips away the fluff and gets down to the meat of what Christianity is about according to the Bible. Along with new believers, many people who have been Christians for years could benefit from it.
2. Tie - Basic Economics by Thomas Sowell and Economics in One Lesson by Henry Hazlett
Covering essentially the same material (though Sowell is more of a monetarist while Hazlett was purely of the Austrian school) these books are indispensable in understanding the root cause of many of our nations economic woes. Both are written for the layman and are make the complex concepts easy for anyone to grasp. I personally prefer Hazlett's book as it's a classic though Sowell's uses modern vernacular as it was written just a few years ago as opposed to in the '30s like Hazlett's and may be easier for the modern reader to digest.
3. The 5 Love Languages by Gary Chapman
I read this book after 10 years of marriage and it opened my eyes to a whole new world of intimacy with my wife. I plan on gifting this book to the happy couple at every wedding I attend henceforth.
4. America's Great Depression by Murray Rothbard
Explains what caused the great depression (large scale malinvestment due to artificially easy credit, in case you were wondering) and how the government intervention of Hoover and Roosevelt in the economy made it immeasurably worse. Something about not knowing history and repeating it comes to mind...
5. The Road to Serfdom by F.A. Hayak
Hayak's classic deconstruction of central planning is more relevant in America today than ever before. Hayak shows that central planning inevitably leads to socialism or its cousin fascism, neither of which are compatible with liberty.
I'm going to cap my list at five books in order to avoid diluting it. If I had an unlimited budget I would buy hundreds of copies of these books, bundle them together and hand them out on all occasions.