I'll compile all the links for a complete set of Fr. Weber's useful collection in this entry.

Go here for links to purchase all four hardbound volumes, #1 from Ignatius Press and ##2-4 from Lulu:
https://sacredmusicus.wordpress.com/2022/01/05/proper-of-the-mass-all-links/

Download links for pdfs:
Volume 1: available from Ignatius Press with receipt showing proof of purchase (details on product info page)
Volume 2: https://drive.google.com/file/d/1dufN-NhkFEguVqchbHtu9Qq_Cr5IcT4K/view
Volume 3: https://drive.google.com/file/d/12c81rZmEz9JjjDfzivFDgCWZZaXs0hH3/view
Volume 4: https://drive.google.com/file/d/1nZHwu-AjKFooi0AIZtAQeVH4N0pHsjFT/view

As proof that I'm not sharing links to the pdfs that are not publicly available, here are the publicly available source websites:
Volumes 2-3: https://www.ccwatershed.org/2020/10/16/indispensable-samuel-weber-propers-english/

Note: the link to Volume 4 at the above webpage is to an incomplete version; below is for the complete version:
Volume 4: https://sacredmusicus.wordpress.com/2021/12/22/proper-of-the-mass-vol-4/