Harry, el perrito sucio by Gene Zion. Recommended reader in Lesson 18 - Limpio y sucio - in the Sonrisas Elementary Curriculum Level 1 and Lesson 24 in Level 2 - Harry, el perrito sucio.
More than anything else, Harry the dog hates baths.So he runs away from home and has the time of his life. But when he comes back at the end of the day, no one recognizes him because he's covered with dirt!
Paperback, 32 pages.