^
Heir to the Empire is the first book of Zahn's "Thrawn Trilogy," which was the beginning of the modern age of
Star Wars tie-ins (before then they were intermittent and lacked the modern interconnectedness) and is generally well-regarded in fandom. Be aware that the three books tell a single really long story a la
Lord of the Rings.
Heir to the Empire is just the first act of the story and isn't a complete tale in itself. Parts 2 & 3 are
Dark Force Rising and
The Last Command.