Just a few off the top of my head -

A Sixteenth Century Anthem Book - Oxford
The Oxford Book of Tudor Anthems
The New Church Anthem Book - Oxford
The Catholic Hymnbook - Gracewing
Graduale Romanum
The Plainchant Gradual
The Anglican Use Gradual
Liber Hymnarius
Simple English Propers - CMAA
A Tallis Anthology - Oxford
Ten Four Part Motets by Palestrina - Oxford
The Oxford Easy Anthem Book
Epiphany to All Saints for Choirs - Oxford
100 Carols for Choirs - Oxford

(Oops, I think I misunderstood that you wanted basic choir repertory.)

Apel's book is a must, it being the most significant scholarly book about chant of the mid-20th century. That said, however, one should bear in mind that much of the content is dated and is no longer 'the last word'. The heir to Apel's book is David Hiley's Western Plainchant, a Handbook, which you should not be without. His other book, Gregorian Chant, together with Dom D Saulnier's Gregorian Chant, a Guide to the History and Liturgy, are also highly informative ought-to-haves. Also, of crucial importance for chant interpretation is Dom E Cardine's Gregorian Semiology. An invaluable commentary on the proper of the mass is Dom D Johner's The Chants of the Roman Gradual. Much historical background and comparative analysis surrounding the repertory of the Graduale is perspicacious and illuminating, though at times reflecting rather subjective and occasionally outdated scholarship. Still, this book is a must.

Also seminal are several books by Fr Columba Kelly, OSB, which may be had from the Scholars' Shop at St Meinrad's Archabbey.

You mention Hymnody: foundational hymnaries are The Hymnal 1940, The English Hymnal, The New English Hymnal, Hymns Ancient & Modern, and The Catholic Hymnbook, had from Gracewing. Though many of these are Anglican they are yet a rich vein of orthodox and historic hymnody which is quite Catholic. An older Catholic hymnbook which is unusually good is the Westminster Hymnal, of which one could likely find only a used copy. A (sometimes) helpful reference book about Latin hymnody is Hymns of the Roman Liturgy, by Joseph Connelly, which gives useful sources and historical notes about many of the office hymns. Its great drawback is one all to common in such Catholic books, i.e., the translations are not in verse and cannot be sung, which means that they have given what is purportedly a more literal translation and sacrificed the substance of the hymn, which is poetry. Translating poetry (hymnody! of all things!) into unsingable prose is, if not intellectually dishonest, then at least intellectually lazy. Still, this book is valuable as an historical reference.
Another very valuable hymnbook to have is The Hymnal Noted, edited by none other than the great Rev John Mason Neale and consisting of the ancient Latin hymns made into English and set to their ancient plainsong melodies. A reprint is available from the Lancelot Andrewes' Press, Glendale, Colorado.

For a psalter, there are two, at least, that you should want to have: 1)The Mundelein Psalter (Grail) set to simple 'modern' psalm tones, similar to the St Meinrad Tones. On the other side you should have 2) St Dunstan's Plainsong Psalter (Coverdale) published by the Lancelot Andrewes' Press, which has the psalms pointed for to sing to the eight Gregorian tones. One further psalter (also Coverdale), for those who may want to incorporate Anglican chant into their repertory of psalmody: The Anglican Psalter, edited by John Scott and available from the Canterbury Press (Lois Fyfe in Atlanta can get it quickly for you). Even if you don't want the Coverdale psalms, this book is chock full of a great variety of Anglican chants that you can use for you own psalmody and propers if you wish. This is but one of many sources for Anglican chant that are available.

If you wish really to jump in somewhat deeply and learn the nature of some aspects of chant scholarship, and come away more learned and with a whetted appetite, you may try a book such as Rebecca Maloy's impressive Inside the Offertory: Aspects of Chronology and Transmission (Oxford). This is an historical treatment of the offertory chants, the guises and various forms in which they appeared in different times and locales while on their evolution into today's offertories. Various liturgical developments and practices appear as embroidery to the main discussion, and the book is sure to become an important one in its field.

Another fascinating book, one which would probably be labelled 'specialised' but is not totally out of reach to the 'non-specialist', is Re-Envisioning Past Musical Cultures: Ethnomusicology in the Study of Gregorian Chant, by Peter Jeffrey (Univ of Chicago). Replete with anecdotes and anthropological evidence, as well as discussing the realities and implications of oral transmission, this book is as exciting as a good mystery - with the bonus that one will have gained really valuable scholarship.

Invaluable contemporary records and comments are to be found in Oliver Strunk's Source Readings in Music History (WW Norton). Multi (not overly large) volumes cover the very early centuries on up to later centuries. You would want to acquire the early volumes.

Another contemporary book which gives a brilliant and beautifully documented panorama of its subject is Christopher Paige's The Christian West and Its Singers: The First Thousand Years (Yale).

I have named just a few, some 'accessible' or 'readable', and some 'heavier stuff'. So, you have an idea of the breadth of chant study and chant lore. There is a plethora of books about the 'wonders' of chant and the 'spirituality' of chant and so forth: 99% of these should be avoided assiduously, for their content is almost wholly of exaggerated and highly subjective hyperbole and outright blather.

A notable exception to the above is Toward a definition of Liturgical Chant, by the Rev Daniel Mark Kirby (priest of the Diocese of Tulsa), which appeared in the Summer 2009 issue of Sacred Music (which you could probably acquire from the CMAA), Vol. 136, No. 2, pp. 5-39. I believe that this is a must read for everyone, especially priests, religious, and choirmasters. It is a sensitive and provocative discussion, well documented, of the spirituality of chant and its role in liturgy and the lives of clergy and musicians, and, ultimately of all who partake of its unique genie. No hyperbole or blather here! Pure intelligence with both feet on the ground. I cannot commend it enough. (In fact, [if any of our CMAA nabobs are following this] I might suggest that Fr Kirby's article should be printed and sold as a separate booklet: it is, in my opinion, highly significant and should reach a wide public.)

Thank you all. I do want choir repertoire, for both the ordinary and extraordinary forms of the Mass as well as Liturgy of the Hours and other devotions outside of the Mass. I am hoping that by picking the brains of this forum I can build a solid library of resources to draw from. Cdpl and other sites I have learned of here are very helpful but I am very much a novice yet, and sometimes I'm not even sure what I'm supposed to be looking for. (I'm not a music director...I'm that parishioner who's constantly agitating for better music). The priest and the music director are moving the parish in a great direction, but they are both insanely busy (the music director has other jobs and a family) so I'm trying to gather resources for them since I have more time to devote to the task.

The Musica divina by Carl Proske, Sammlung ausgezeichneter Kompositionen by Lück (both found on IMSLP).

Frogman noel jones... thank you as well. I will search out those books. I've only been singing chant since September and although I'm beginning to "get" it, I would greatly appreciate more in depth study.

I would also include the Simple English Propers.