Frazer, Sir James: The Golden Bough [Abridged], v1, 28 December 2009.
 

Sir James George Frazer. [1854–1941]
The Golden Bough: A Study in Magic and Religion
Abridged Edition (1922)

Originally published in 1890, this is an abridgement by the author. It is only (!) 410,000 words long.
Sir James Frazer discusses and compares similar magical and religious practices across the world, suggesting that there are many common elements. He surmises that there is a pattern to the development of human beliefs: primitive magic evolves into religion, which may evolve into scientific belief.
It is also strongly implied that there is nothing new in Christianity, other than the personality of Jesus of Nazareth. All the other features also occur in various mystery religions, e.g. the cult of Osiris and Isis.

The source was Project Gutenberg, with a number of additions and corrections from my paper copy..

Incidentally, in George Eliot’s “Middlemarch,” one of the characters, Mr Casaubon, is busy writing a “Key to all the Mythologies,” which remains uncompleted. This is the work that he might have written, had he not been an intellectual coward.