I checked with the Minnesota Center for Book Arts regarding Diane Longley's book on polymergravure, but they no longer keep it in stock, so I guess you'll have to order it directly from her website in Australia. With all this talk of gravure, I dug out another couple of books that I bought years ago that helped me at the time.
One is "Photopolymergravure - A New Method" by Eli Ponsaing, who is regarded by many as the pioneer of polymer photogravure. It's published by Borgen in Denmark. The book is in Danish with some English translation and plenty of illustrations. The illustrations are easy to understand which is fortunate, because the translation isn't the best.
A better book is "Polymer Photogravure - A New Method for Photographers and Graphic Artists" by Taneli Eskola and Kari Holopainen. Published by the University of Art and Design in Helsinki. This book makes the most sense of the process, along with Longley's. It takes you through the history and the process as well as many printmaking techniques. Unfortunately, my copy was printed in 1996, so there are no digital references at all and I don't think it's been reprinted. Both books are available through Photo Eye.