I think the catch line "The inspiration of a whole generation of artists..." (which is from a commercial promoting these books) is right on the mark.
The difference from this book and the 'Hope' and 'Jedi' books, is that the script isn't included in this one, so you feel that you get more for your money. And also, at least in my mind, Empire offers more interesting environments and imperial machinery than the two other original films, just take Cloud City as an example.
The last 16 pages of the book are solely based off of the work made for the special editions, showing some of the work that went into the hoth monster scene and the cloud city scenes.
Altogether I love this book, I often use it as an inspirational source in my own work, and I strongly suggest it for anyone who's even remotely interested in Star Wars artwork. A lot of the pictures and sections have been commented, giving a rich look into how the people behind the movie went about their business (pun intended :)
May the force be with you... always