He says that "As far as I am concerned, these are the four most important English books for those who love chant and want to have a firm background in the heritage of chant in the Western tradition."
* David Hiley, Western Plainchant: A Handbook (Oxford: Clarendon, 1997). This is a truly monstrous compendium that presents the main data on western plainchant. This is an excellent reference book. It's not necessarily the kind of thing that one sits down and reads cover to cover but if you can handle it, you'd come out way ahead.
* John Harper, The Forms and Orders of Western Liturgy From the Tenth to the Eighteenth Century: A Historical Introduction and Guide for Students and Musicians (Oxford: Clarendon, 1991). This is one of the most accessible--yet serious--books to lay out the various complexities of the Western rites. It's an overview of the Kalendar, Medieval Liturgical Books, the Mass and the Offices. The focus is on England and English rites and includes chapters on the early Books of Common Prayer as well as the Medieval and Catholic materials.
* Cyrille Vogel, Medieval Liturgy: An Introduction to the Sources, (trans. and rev. William Storey and Niels Rasmussen; Washington, D.C.: Pastoral, 1986). Also an excellent reference, this is a hard-core text that looks at the main sources of the Western liturgies from the earliest times up to the Council of Trent. To know chant, you've got to know about the liturgies that it developed in relation to. If you've ever wondered about the differences between a pure Gelasian sacramentary and a Mixed Eighth-century Gelasian, this is the source for you. Especially helpful for reconstructing early medieval rites before the rise of true Missals is the summary of all 50 Ordines Romani. Early medieval liturgical books only had the parts for the participant it was written for; the ordines were the instructions on how everything fit together.
* Andrew Hughes, Medieval Manuscripts for Mass and Office: A Guide to their Organization and Terminology (Toronto: University of Toronto, 1982). If Vogel and Harper left you itching for more, Hughes is your passport to the next level. Hughes gives you the theoretical and practical information that you need to confront medieval liturgical manuscripts on their own terms. That is, when you confront your first sacramentary, how would you even begin to find what you're looking for and not fall into the most common mistakes that modern readers make with medieval texts. Of course, to get the most out of this book you'll need to brush off your Latin and start diving into some real texts.
Once you've gotten a good grounding in these it's time to go hunting for bear--and here's one of my favorite new sites. Look for your favorite chant text and music here.
LutheranChik adds that she found "a website with lots of Coptic liturgical chant, hymns, etc."