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 Youtini Expanded Universe Ranking system is a 1-5 scale that ranks EU book quality, difficulty, and importance. Books closer to Level 1 are easier to read and understand, more important, and of higher quality.
Star Wars: Galactic Atlas is a reference book with full-color illustrations by Tim McDonagh, it covers the first seven Star Wars films, as well as Star Wars Rebels, Star Wars: The Clone Wars, novels, and comics.