I'm not obsessed with the Star Wars films, although I did enjoy them. But the great thing about this book (and the other "Art of Star Wars" books) is that you don't have to be a huge fan to appreciate the conceptual artwork. As an artist, I consider this book to be one of my favourite reference books. It shows much about what goes into the preparation of a film. This book IS rather fan-oriented, and lacks some critical information such as the sizes of sketches, and other technical information about the paintings and drawings which would have been helpful to aspiring artists & designers. However, it is never-the-less an almost daily source of inspiration for me as I do my own futuristic art.
As for the script, I could take it or leave it. I've never read the whole thing, only bits here and there. What amuses me is how the script is subtly different from the movie. It's clear that although Lucas has a great mind for stories, his writing is only adequate. The script is rather awkward. If you want a good story, see the movies: the script doesn't do them justice. But for great behind-the-scenes production and concept artwork, see the book.
I would also recommend getting the modern 1995 re-issue of the book, not the 1979 version, if durability is an issue, because mine, like so many others I see here, has fallen apart.