What Are Propers?
If you're not sure what propers are, you're not alone. In short, the propers are what, since the second Vatican council, have been replaced by hymns or songs. The propers are what should be sung. They are the parts of the mass that change with the day that includes the introit (usually replaced by entrance hymn/song), the gradual (aka responsorial psalm), alleluia, offertory antiphon (usually replaced by offertory hymn/song), and the communion antiphon (yes, you guessed it, usually replaced by communion hymn/song).
Full Propers Books in English
If you'd like some options in English, check out these great ones below. They range from simple chant to full SATB, all containing the Introits, Graduals, Offertories, and Communions.
Specific Propers Books in English
Here are some other great options that are written for specific parts of the mass. Just as there are responsorial psalm books that have all the options for the 3 year liturgical cycle.
The Rossini Propers are a simplified version of the propers for the mass (extraordinary form), arranged by Fr. Carlo Rossini, aimed at allowing more novice choirs to sing the propers. These are not meant to be a permanent option to the Roman Gradual, but a temporary one while said choir builds their skills up to singing the propers from the full melodies of the Gradual.
This section contains modern re-arrangements of the Rossini propers which can be hard to read since multiple propers are all displayed on single pages. These have 2 pages to print each Sunday, which makes them a little more paper-consuming (except if you're using an e-reader of course) but a lot easier to read and follow. More will be added as I finish them for my own schola.