Consumer note: this post discusses the second book in Roger Zelazny's Amber series. Skip if you want to avoid spoilers for the first book, Nine Princes in Amber.
Which reminds me: I noticed that the back cover of this omnibus volume lists the first novel as Nine Princes of Amber. That's kind of a sloppy mistake for a publisher to make.
Anyway: when we left our hero, Corwin, he had just escaped from Amber's dungeon, vowing revenge upon his imprisoner/usurper/brother, Eric. That's the story here: Corwin's scheme is carried out with the help of his old ally from past conflicts, Ganelon. He encounters another brother, Benedict, who's distrustful, but agrees to keep Corwin safe from Eric's clutches, at least for a while. And there's the mysterious young Dara, who spins a story believable enough to convince Corwin she's just an impetuous youngster. Again: at least for awhile.
All this is set in the Amberian cosmology, which is: Amber is the only "real" world, all others, including our own Earth are merely shadows. Corwin and his kin have the knack of journeying between Amber and shadow worlds.
There's lots of room to quibble with this arrangement. We discover that the laws of physics differ enough between shadows to make (for example) gunpowder unworkable as a firearm explosive/propellant in Amber. (The source of the book's title.)
But you can only play that game so far before you make life itself impossible. How do you avoid accidentally bouncing into one of those life-hostile shadows?
Worse, it seems that there are an infinitude of shadows. How do you get to the precise one you're aiming for?
Maybe I missed something. Or maybe an explanation is forthcoming. I'll let you know if I find out.