BBY/ABY is used to position items along the Star Wars Timeline. BBY refers to Before the Battle of Yavin. ABY refers to After the Battle of Yavin. The Battle of Yavin in Episode IV: A New Hope occured in 0 BBY/ABY.
The Maverick Moon is a children's storybook released in 1979 and published by Random House. It was written by Eleanor Ehrhardt (uncredited in the book itself) and illustrated by Walter Wright. It is one of three such children's Star Wars storybooks released that year, along with The Wookiee Storybook and The Mystery of the Rebellious Robot. It takes place at an unspecified time after Star Wars: Episode IV A New Hope.