I liked the seven books of amber and other works by this author, so I tracked this one down and gave it a try. I did not like it. It's book two of a trilogy, and lacks some background. Second, it's young adult orientated, a novella, and alternates three main characters. That means the plot isn't deep or extreme in any fashion, except possibility the 'what if' aspect involving alternate worlds, but again, as a long time sci-fi reader, I've seen it done better elsewhere (Chalker, Handleman, Sliders the TV show, etc).
This might be a great book for grades 6 - 8, but adults should consider reading elsewhere.