The Golden Bough
by Sir James George Frazer

This is a classic on the development of religion and civilization.  The section on Mithra might help Christians understand the origin of some church ceremonies still practiced.  It also tells us the reason Christmas is on December 25th (the early Christians really didn't know what day Jesus was born on).  Easter is another Christian day where even the name was borrowed from earlier religions.  A fascinating read for those interested in comparative religion or religious history.  Rated four stars on Amazon.com.