One of Eliot's notes to "The Waste Land" acknowledges an influence
from Jesse Weston's "From Ritual to Romance."  An online copy is at
http://world.std.com/~raparker/exploring/books/from_ritual_to_romance.html

Weston footnotes works by Franz Cumont and I just found a new online
copy of one Cumont's works.  This is at:
http://www.sacred-texts.com/cla/orrp/index.htm

The description there is:

The Oriental Religions in Roman Paganism
by Franz Cumont
[1911]

This is a very detailed review of what is known about the eastern
religions which were absorbed into Roman Paganism during the late
Republic and Imperial periods. Cumont, who also wrote The Mysteries of
Mithra was a Belgian scholar who specialized in the study of ancient
religion. Topics include the cults of the Magna Mater, Cyblele, Isis,
Astarte, Mithrism and Zoroastrianism, as well as a study of astrology
and how it became part of Roman beliefs. Roman Paganism started out as
a system of animist beliefs with a simple pantheon of gods and
goddesses. Contact with these diverse near-eastern religions added a
sophisticated system of ethics, ritual and cosmology and a host of
additional deities.