The Parish Book of Chant (2nd edition): parishbookchant_2nd (downloaded from musicasacra.com . Much more is available directly on musicasacra.com , especially on this page: Musica Sacra – Music pdf )
The Gregorian Missal, 1991 edition.
offertory verses (simple) from the offertoriale triplex
Offertory Proper Triplex long
Resources to start the path from the Bible to a chanted liturgy, in English:
Father Weber’s music – Chant in English
Lumen Christi series (Chant in English)
Two sites with a lot more resources:
Musica Sacra Church Music Association of America
Corpus Christi Watershed : more focused on the “Extraordinary form” using the 1962 missal.