Modal Harmony Book Suggestion

The title pretty much says it all.  I'm interested in studying early medieval polyphony, as well as monophony, and I'm having a great deal of difficulty finding resources.  Any texts or online resources that can be suggested would help me tremendously.

Thanks in advance!

For Renaissance and earlier, you should look over book by musicologists, like Knud Jeppesen's "Counterpoint - The polyphonic vocal style of XVI century".

For styles using a fugal structure, you can search the titles quoted in the first chapter of Alfred Mann's "The Study of Fugue".

Ah, also you can search at Wikipedia, the list of resources (the end of the page) of each composer/style you wanna learn. Try out "ars nova", "ars antiqua", "rhythmic modes", "organum" etc.