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.
Meet Chopper--the rebels' grumpy old astromech droid. He may have a bit of an attitude, but there's no droid the team would rather have with them in a tight spot. This Level 1 early reader retells one of Chopper's exciting adventures with his rebel friends.