Pet mouse tells stories to his young girl owner From the 1960s or 70s. A young girl recovering from rheumatic fever gets a pet mouse. She discovers that he can talk, and he tells her stories of his past adventures. I think the book title is the name of the mouse, but it was an unusual name.
Stump the Bookseller
New format, same book stumper solving.
Tales of Mr. Pengachoosa, by Caroline Rush. (He’s a hamster though.)
“Eight short tales, told by a pet hamster to his owner, a little girl, convalescing after rheumatic fever.”
From the description that might be it.