HENRY AND MUDGE AND THE SNEAKY CRACKERS by Cynthia Rylant

Henry and his big dog, Mudge, loved watching spy shows, so when there was a spy kit on sale, Henry bought it. He and Mudge went spying and they ended up being friends with a boy and his dog who, like Henry and Mudge, also loved watching spy shows and being spies.
- T. Lyn