Edward Carpenter 
Pagan and Christian Creeds: Their Origin and Meaning [PDF ebook] 

The book was a great read for understanding how the different religions all borrowed from one another. The author gave great anecdotes from both the Pagan and Christian perspective that covered egyptian, Roman, and many other faiths that were both poly and monotheistic in nature.
The author did a very good job of comparing the rituals and symbols of ancient religions and how they relate to modern day Christianity. This was a well thought out, organized, and researched book.