There is a website for
Building the American Republic (
https://press.uchicago.edu/sites/bui...lic/index.html) which gives links for downloading both volumes in ePub, PDF, and Kindle formats, as well as a link for reading the books online. If I understand the above site correctly the intention is that digital versions of the volumes will always be free.
The e-books should also be available for free at "major e‑bookstores." For example, in addition to Amazon as linked to by GtrsRGr8 they are available at Kobo:
Kobo UK
https://www.kobo.com/gb/en/ebook/bui...ublic-volume-1
https://www.kobo.com/gb/en/ebook/bui...ublic-volume-2
Kobo USA
https://www.kobo.com/us/en/ebook/bui...ublic-volume-1
https://www.kobo.com/us/en/ebook/bui...ublic-volume-2
For other countries change the "gb" or "us" to whatever code your location uses.