Is there a good, inexpensive book (that is currently in print) with a collection of "standard" sacred polyphonic chants to be sung at, e.g., Offertory or Communion?
Geremia, are you looking for a collection of motets? Oxford's "European Sacred Music" has 54 of them, and you can get a used copy for under $10. Admittedly, not everything in it is suitable for Catholic use, but most is. I expect you can view the table of contents at Amazon's listing for the book.
I second the frog music book. All the best stuff, and lots of it.
As for beginner stuff, I just bought 2 copies of Rossini's "Canticum Novum;" I know you didn't want anything out-of-print, but you can just download it for free and print individual pieces as needed, which might work better than a book anyway.
There is a steadily growing collection of Heinrich Isaac works at CPDL, from his monumental opus "Choralis Constantinus," beautifully edited and engraved by our own MusicaSacra colleague Corinne Cooze. I recommend them highly.
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