Boo is now the star of the book, "Boo: The Life of the World's Cutest Dog," which takes fans inside his adorable canine world. "Good Morning America" asked Facebook fans to send in the best caption for these photos of Boo. Without further ado, here are some of our favorites.
"Sergeant Boo, reporting for Duty" -- Evan Haggerty
"Now you see me, now you don't!" -- Jill Brummer Wolf
(Photographs by Gretchen LeMaistre/Courtesy Chronicle Books)