This is all a result of using music downloads - something I love because otherwise I wouldn't even known about much of this music, but which can also be a problem. Ideally, all the music that a young choir needs should be in a single volume.
I didn't know until today that such a book does in fact exist. It is the Catholic Choirbook, as put together and edited by Noel Jones. He did a fantastic job in choosing the contents. Here is the core of the Latin and English music (motets) that you need for singing at Mass, along with a fine selection of hymns. These are not the propers of the Mass but rather music you need for preludes, offertory anthems to sing after the propers, during communion, and otherwise. It is a great selection. The book is 381 pages, and he has used his experience - and that of William Mahrt's - to select the best of the best: the material that is most accessible and most useful for the Catholic choir.
You can read more about it here. And you can order the book here. Everything in it is produced using Creative Commons, so you can photocopy without fearing lawsuits. But much better is that fact that everything is nicely paginated. Mostly, I think many choirs could just replace their current chaotic system with this one book.
This is certainly what I would do if I were starting over. Oh, and the introduction is by Mahrt himself. This is really a wonderful book, certainly worth having if only to save you the time, annoyance, and frustrations of having to tackle the internet every time you need something to sing.