Ralph S. Mouse is the third in a children's novel trilogy by Beverly Cleary. It features Ralph, a mouse with the ability to speak, but only with certain people, who tend to be loners. It was first published in 1982, illustrated by Paul O. Zelinsky.
Reading her books was one of the best parts about my childhood. I read my first Ramona book when I was in third grade (though I forgot the specific book). I feel in love with characters like Ramona, Beezus, and Henry, and I ended up reading every book in the Henry/Beezus/Ramona series that was published at the time (which ended at Ramona Forever, though there's another book, Ramona's World, that was published after I outgrew the series). Though I haven't touched a Ramona book in about 22 years, I still vividly remember the characters and the Alan Tiegreen illustrations. Beverly Cleary also wrote another series of books that I love featuring a hotel-dwelling mouse named Ralph that was given a toy motorcycle. I still have my copies of "Ramona Quimby, Age 8" and "Ralph S. Mouse" somewhere.