As a teenaged Dune fan I remember finding this book in a bookstore, being intrigued, and reading more. However, I did not buy it, and grew to curse that decision as it went out of print all too soon. That was all 25 years ago and more. Now I have found a secondhand copy in a used bookstore for a song - the proprietor said it lasted not a half a day on the shelf when I bought it. I told him he was selling it too cheap. He said he wont next time. Sorry about that, if you are the next time.
So now I have a copy again...and it is all I remember and more. This was assembled with Herbert's blessing post God-Emporer of Dune, but pre- Heretics and Chapterhouse. It is a beautiful compilation of "facts" about the Dune universe, written like a novel in terms of readability (which should not be too surprising - its basically high-class fanfiction, after all). It is full of little nods and in-jokes (like an anagram of Jack Vance's full name being used for a character in an entry). Its unique, and I love it.
This book is out of print, and unlikely to be reprinted, at least in its current form - and I imagine it would be easier to write a new volume for the new canon rather than attempt to rewrite this. If you love the original Dune, then this book is certainly worth having, if you can find a copy at a reasonable price.