Basil of Baker Street by Eve Titus, 1958.
Some people may recognize the title character from Disney’s The Great Mouse Detective. This is the original book that the Disney movie was based on, although the movie was quite different from the original book.
The part that is the same is that Basil is a mouse who lives in the house of Sherlock Holmes at 221 B Baker Street. He and some other mice have built a little town they call Holmestead in the cellar. Although Sherlock Holmes is unaware of Basil’s presence, Basil studies his methods and copies them, becoming “the Sherlock Holmes of the Mouse World.” Like Sherlock Holmes, Basil also has a narrator for his stories, his best friend and fellow mouse Dr. Dawson, and they solve cases for their fellow mice in trouble.
In this book, Basil’s skills are put to the test when he is hired by his mouse neighbors to find their two missing daughters, a pair of twins named Angela and Agatha. The twins have been kidnapped, and soon a note arrives, saying that all the mice in Baker Street have only 48 hours to evacuate their home if they ever want to see the twins again. A gang called The Terrible Three want to use Holmestead as their headquarters. With only two days to solve the case, Basil and Dr. Dawson set out to find the gang and rescue the twins so that they can keep their wonderful home!
Basil of Baker Street is actually the first book in a series about Basil’s cases.