For many years, I've heard of the Offertoriale of 1935, which contains Offertory chants plus full verses with Gregorian melodies. I had never seen a copy despite filing alerts with every book service I knew. An email requesting this arrived about once a fortnight. I was beginning to despair of ever getting it online much less in print again, or even seeing one in real life. Then a gift arrived from Arthur Connick: a perfect copy ready for printing. I believe that this addition now completes the digitized library of chant for the Roman Rite. These verses are particularly wonderful for any choir that works up the offertory chant and wants to extend it through the entire time in which the choir is singing. This edition is applicable to both the extraordinary and ordinary forms of the Roman Rite. It is in print for $15. (Interesting, isn't it? I would have paid hundreds or thousands for a copy only last week.)
The confusion over what precisely constitutes the Offertory Chant in the ordinary form is due to its having been left out of the printed Missal, which contains only the parts that are applicable to the celebrant and responses. The Offertory chant is for the choir alone, and it, but not the verses, appears in the Graduale Romanum. The entire list of propers in the new calendar are available for download.
Here is a download option for the Offertoriale.
