Number four in Roger Zelazny's Amber series. I repeat my usual warning: Avoid reading further if you haven't read one through three.
Where we left off: Protagonist Corwin, with buddy Ganelon and brother Random, just made a remarkable discovery about the nature of Amber. It's not quite what they thought it was, and that may well make the long term survival of that unhappy world less likely. Or, now that they know about it, more likely. Who knows? Yes, it's one of those fantasy series that seems unsatisfied with the fantasy-rules laid out in the first book, and seeks to alter them.
Why, it's almost as if Zelazny was making this stuff up as he went along.
Anyway: it's the usual mix of intra-family betrayal and lies, hallucinogenic trips to alternate universes, a quick visit to "our" earth, violent clashes with both fantastic beasts and humans. It ends up with a thrilling battle between good and evil. And, oh yeah, a shocking twist ending. No spoilers here, although you might see it coming if you look at the title of the book.