I've been fielding an unusual number of emails recently on where to begin with chant. It can be a difficult question. It is like asking: where do I begin with Catholicism?
Do you know what I mean?
So I've done some serious thinking about this issue, listening to and looking at chant CDs for days, and decided that the chant hymns--the most familiar parts of this music for 1000 years--are really the best start, because they give the singer practice, form the starting point for the real-world schola, and they involve the people in singing.
I don't mean to slight the many recordings out there, but, overall, I have to give the prize for the best starter CD to O Lux Beatissima, as sung by Cantores in Ecclesia and produced by OCP (not a typo). It uses both high and low voices, offers some simple ordinary settings, and provides excellent renderings of many popular chants. This is a great starting point.
For music, the answer is easy. The Parish Book of Chant provides a tutorial and 70 hymns plus 11 ordinary settings plus ordos for the old and new forms of the Roman Rite plus a pronunciation guide plus English translations. That's all the features you need to go a very long way in learning the chant.
The next steps include the Gregorian Missal, which is produced by Solesmes, though the book is so expensive and margins are so thin that there is a real opportunity her for some U.S. publisher to publish a similar work of propers. The low $29 price from Amazon won't last, not when GIA and others are charging $40+.