When researching recovery from wounds in World War One, the best sources I found were in the Biomedical Library at the university where I work - they actually had some books from the period about physical therapy, which gave me an idea of how treatment might go. I also used letters, but that was very, very piecemeal so far as useful nuggets of information went.

I wonder if there are any memoirs from Civil War amputees?

Maybe JSTOR? I have access if you need anything downloaded: https://www.jstor.org/action/doBasicSearch?Query=amputee+civil+war+american