I've never understood the "love it or hate it" reaction that so many people have to this book.
Zelazny was interviewed by Randall and Hamilton for the express purpose of creating this little book. He described the people, the places, and the history of Castle Amber in more concrete detail than you get from the 'Amber' novels, and his descriptions were then "fleshed out" a bit with original prose and illustrations to create 'The Visual Guide to Castle Amber.'
Your guide is Flora, one of the characters from Zelazny's 'Amber' series, and her narration takes you on a floor-by-floor tour of the Castle. Each floor and many rooms of the castle are mapped and thoroughly described. There are also Trump illustrations, along with an incomplete family tree (the 'Visual Guide' appeared before all of the 'Amber' novels were published). Quite interesting, fairly well-organized and well-presented, and authorized by Roger himself. What's the fuss?
The way that fans react to this book has always been amusing to me. Depending upon the individual, the 'Visual Guide' is either a foul blasphemy that defiles THAT WHICH IS THE ONE TRUE AMBER, or else it's a definitive "last word" that was so important that Zelazny assumed flesh-and-blood form to deliver it to the unworthy masses.
I look at it this way: The 'Visual Guide' is ONE VIEW of what Amber might be like, informed by Roger Zelazny and expanded by a couple of self-professed Ambermaniacs. I've seen better, I've seen worse. Generally speaking, I like the maps fairly well but most of the Trump illustrations are run-of-the-mill. I enjoyed leafing through this book quite a bit, and I'd say it's worth reading if you're an 'Amber' fan like me, but I don't go back to it time and time again the way I do with Zelazny's fiction.
Taken for what it is, the 'Visual Guide' is a nifty piece of fan art and fiction, no more and no less.