This book is at least 25-30 years old. It was about mice/ rats in a hospital after hours. There was one character with a broken tail from a trap and they had their own hospital ward with little beds for the mice/rats. The imagery was quite dark, not typical for a kids book.
Stump the Bookseller
New format, same book stumper solving.
Could be Emergency Mouse by Bernard Stone: A young boy who is hospitalized discovers that a group of mice operate a hospital of their own in the wall of his room. Illustrations are by Ralph Steadman, who’s well-known for collaborations with Hunter S. Thompson, so could be the “dark” imagery you’re remembering.