The Ancient Mysteries : A Source book
Sacred Texts of the Mystery Religions of the Ancient Mediterranean World
Marvin W. Meyer (Editor)

What you'll find:

A sourcebook of extended quotations from ancients, all dealing directly with the Pagan mystery religions.

Who you gonna trust?  The ancients. Believing scholars shade the facts in favor of the myth.  Non-believers exaggerate and make up facts and connections as a way to attack the church. 

So who are you going to trust?  That's up to you.  I trust the ancients—people alive back when Christianity began, and before. That's what this book is about.

This is a sourcebook, a collection of primary documents—excerpts from ancient authors who wrote about Pagan religion and early Christianity.  It's a great collection, with the original text of most of the standard ancient references to the pagan mystery religions.

This is a powerful book. You'll discover firsthand, right from the pens of the ancients themselves,  that Dionysus came to earth "incognito, disguised as a man"; that Pagan Gods died and were reborn with the meaning that "the God is saved, and we shall have salvation."; that pagans had initiation ceremonies seen as "a voluntary death", sacred meals shared with the God, ceremonial washing, Pagan miracles, a Godman who changed water into wine, and a Pagan version of the great flood.  And much more.  

An important book that no serious student will be without. Highly recommended.

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Early Gentile Christianity and Its Hellenistic Background
by Doctor of Divinity Arthur Darby Nock

You'll find:

	the leading non-borrowing scholar- apologist admits deep similarities between the Pagan mystery religions and Christianity.
	The canonical believers' reasons why each and every one of those similarities doesn't count.

First published in 1928 and reissued and updated in 1964, this is the canonical refutation of the late 19th and early 20th century scholarly claims that Christianity borrowed from Paganism.  This essay is widely cited as an authority, "Dr. Nock has refuted the German School. . .", and the arguments Nock developed here are the same ones believers use today.

The Eucharist ... is in line with contemporary mysteries, which purported to represent the sufferings and triumph of a god, in which his worshipers sympathized and shared....The Eucharist is a mystery, as mysteries were then understood, and Christianity, the heir of Judaism, has also an essential spiritual continuity with Hellenistic religion.
[pg 72]

Nock was also a committed Christian, a Doctor of Divinity who wasn't about to admit Christianity borrowed from Paganism, so for every similarity he comes up with a reason the similarity doesn't count.

The 1964 Harper Torchbook edition is expanded with Nock's later thoughts and arguments. 

It is out of print, but often available used through Amazon

The Mystery-Religions
A Study in the Religious Background of Early Christianity
by Samuel Angus

What you'll find:

	A famous and much cited account of the Mystery Religions.  Written in the 1920s, the book is still in print.
	Organized by feature- of -religion, so you get an overview of the mysteries rather than a detailed look at each of them.

Hellenistic Mystery Religions
Their basic ideas and significance
by Richard Reitzenstein (1861 - 1931)
translated by John Steely

What you'll find:

	as far as I can tell this is the only English translation of this highly famous, highly influential, poorly organized, scholarly analysis of the Mystery religions.
	The good news: Reitzenstein supports his arguments with extended quotations from primary ancient sources. The bad news is, get this: the quotations are not translated into English, so unless your Greek and Latin are shiny, a very frustrating book.

This English version was published in 1978 and is now out of print. You can sometimes find it used at Amazon.com

Isis and Osiris
in Moralia V
Loeb Classical Library #306
by Plutarch

This is the same Plutarch who wrote Plutarch's Lives.  Like Solon, Plato and Pythagoras before him, when he wasn't biographying Plutarch traveled to Egypt and studied the mysteries of Isis and Osiris—probably even got initiated (though he doesn't say for sure).

Isis and Osiris, at just over 90 pages, is modern scholarship's main source for the goodies on one of the ancient world's big name Pagan religions. 

This Loeb translation is pretty easy to read. And fun. You'll discover "accounts of the dismemberment of Osiris and his revivification and regenesis" [Isis and Osiris, 365]—His death and resurrection!  

Wow.

Be careful, there are a bunch of P's Moralias in print at Loeb and elsewhere. For Isis and Osiris, you want number V, which is Loeb #306.

And the good thing is, you don't have to believe me, you can read it for yourself.

The Jesus Mysteries
Was the Original Jesus a Pagan God
Timothy Freke, Peter Gandy

	A popular book introducing Christianity's Pagan Origins, arguing that Christianity developed from a Pagan Mystery Religion (a thesis advanced by mainstream scholars in the early 20th century).  Well written, well organized, easy to read.
	But this is amateurish scholarship, almost all from secondary sources.
	For some reason Freke and Gandy try give the impression the Pagan origins was something they discovered themselves—which is nuts, and which cripples their credibility.  Still, the book is concise, focused and readable.

Whether or not you accept the conclusion, the early chapters laying out the well know basic facts about  Pagan / Christian connections will change your understanding of Christian origins and the history of Western thought.

 
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Interesting, but not essential to the Pagan Origins question.