Three ideas come to mind:
I know that, at least in my local library system, books that are not used for a certain length of time are eventually discarded. I have been told that some of the practically never used books have been saved from the discard heap simply because I have taken them out of the library. These books are often obscure older Catholic publications.
Idea #1: Call or somehow contact the Boston College library and inquire about the use of the book. What is the most recent date that it was taken out by a patron? If it hasn’t been used in 10 years, they may allow you to purchase the book in the light of its sentimental value for you. Make sure they understand the sentimental value.
Idea #2: If the book has been used but only seldom and the length of time needed to pass before it goes to the “discard” pile hasn’t passed, ask that a note be attached to the library’s information system. This note would indicate that, should the book end up being discarded, they should contact you to see if you would like to have it. I worked in a library here in Masachusetts many years ago and I do recall the librarians complying with that request. (Of course that was before digital records.)
Idea #3: I did a brief search myself, CampusI and such, and I imagine you know all thes earch engines. Just making sure you did search the rounds of used Catholic book stores on line-- “Preserving Christian Publications” is the topmost I would consider. They have a lengthy list of books and it is always changing, so if they don’t have it now, they could in the future.
I wish you luck while you search for your dear Uncle’s book. Let us know if you meet with success!