When I was a child, this would have been sometime between 1990-1995, I borrowed a fiction chapter book twice from my local library that I cannot remember the title of. At the time, the book was old, but I was a kid so it may not have been THAT old. I think it may have been written sometime around the 1950s-70s. It was a story about a boy who takes care of a squirrel. I think that there was a red squirrel on the cover, and the title of the book may have been the name of the squirrel but I’m not certain. I think the book had a bittersweet ending with the squirrel going back to live in the wild. I’ve not been able to find it on a google search so perhaps you can help!
Was it The Adventures of Chatterer the Red Squirrel by Thornton W. Burgess, 1915? You should be able to read it online. (He wrote a LOT of such chapter books. Many begin with “The Adventures of…,” but there are others. The total is well over 150 books.) See here: http://www.thorntonburgess.org/Books-Written-by-Thornton-W-Burgess