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.
Heir to the Empire is essential reading material because without it, the rest of the Expanded Universe might not exist. In this book, Zahn manages to construct a new type of Star Wars villain in Grand Admiral Thrawn who defeats his enemies with superior intellect that is every bit as overwhelming as Vader’s Force choke. Luke, Han, Leia, and the entire cast of the Original Trilogy make their triumphant return and must find a way to deal with this new threat who is poised to bring back the Empire even stronger than it was before.
Timothy Zahn has arguably crafted some of the most important books in the Expanded Universe and they all started with Heir to the Empire.