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.
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.