These are pdf copies of
Regondi's New Method for the [English] Concertina, and
Regondi's Rudimenti del Concertinista or A Complete Series of Elementary Progressive Exercises for the [English] Concertina.
Allan Atlas has the original copies. He sent me photocopies and I have scanned them. The links below have each tutor is broken into multiple files of at most 9M. Each book as a single file is available on my Google Drive page.