Gregorian chant. Saying the words can put some people into a state of euphoria and make others have apoplectic fits.
For those who are used to standard music notation, chant notation can seem like learning Latin when you know Spanish - it looks pretty familiar but you can't just jump in and start singing.
First of all, learning chant is actually much easier than learning standard notation. There are only two clefs and no "key of ___". There are only four bars. There aren't any sharps. The one area where chant can get complex is in the actual interpretation of the notes.
If you want an in depth history of chant before you get down to business, we suggest the book Gregorian Chant by Willi Apel. This book covers the entire history of chant both in the Latin rite and also contains information about Ambrosian and Old Roman chant.
To start your journey, the best book we have found for learning chant is The Square Notes Workbook. This book was designed to teach you chant from the ground up in simple lessons. The book teaches the different types of notes as well as techniques for singing the texts without making them sound monotonous. The Square Notes Workbook also contains questions at the end of each lesson that are great if you are trying to teach a class or even just make sure you have learned the material yourself.
If you want a book that is a little more advanced, Beginning Studies in Gregorian Chant is a good book to look at. This book was written as a class book for Solesmes - the home of the modern chant revival.
There are two CDs we also recommend for learning the basics of chant. The first is Learning Gregorian Chant which is an introduction to chant interspersed with sung samples by the monks of Solesmes. The second is O Lux Beatisima which is one of two (2, dos, duo)! chant products from Oregon Catholic Press. This CD has a great selection of common chants that could be the start of a parish repertoire.
Once you have a basic understanding of chant, it's time to take a look at some real chant hymnals. The booklet Jubilate Deo was published by the Vatican after Vatican II as the basic list of chants that every parish should use so I'm sure you are all familiar with it and use it regularly ;-). Unfortunately, this booklet takes the chant and translates it into modern notation.
The Adoremus Hymnal, which comes in congregation, choir and organ editions, contains a large selection of chants for the Ordinary of the Mass in chant notation as well as a large selection of Catholic hymns in a more traditional vein for Mass. We have used this hymnal in our parish and except for a lack of dynamics markings and the strange inclusion of some harmony parts that aren't actually for voices, the book has worked well.
If you would just like to learn the Ordinaries of the Mass and a selection of other chant hymns, the Kyriale is for you. This hymnal is a great supplement to your standard parish hymnal if you would just like to start adding chant to your Masses. This hymnal was also published by Solesmes.