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.
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.