David Hiley's book Western Plainchant: A Handbook, published in 1993, remains the definitive entry point to chant scholarship and reference source for myriad subjects in the field.
Hiley's book replaced Willi Apel's 1958 book Gregorian Chant by virtue of its inclusion of the latest scholarship in the field, but also because its scope is so broad. Starting with an insightful look into the basics of liturgical placement and chant genres, it goes on to explore the primary sources vital to scholarship in the field. There is also the discussion of history through the lenses of notation, early music theory, persons and places of import, the influence of other non-Gregorian repertories, and ending with a discussion of the work of Solesmes.
The book, whilst remaining a reference, is also eminently readable. It assumes a non-specialist reader, but doesn't shy away from the complexities of the questions that arise in a study of the material.
Why bring this book up twenty years after its publication? Because it is available in a free pdf download from the University of Regensburg, where Hiley works. (Caution, the file in the given link is very large!)
A category search on the University's publications repository also yields a large amount of English-language scholarship, available for download.
