Liber Usualis in Latin with introduction and rubrics in English
This is really the mother of all Gregorian chant books.  The introduction in English is a very good exposition of chant reading and singing technique.  And it is upon this platform I have purchased other books of chant that are still sung to this day in various parts of the world.  There is really nothing to compare with it since it is one of a kind.
Rating: 5 stars