Number 7 of 10 in my attempt to read Roger Zelazny's Amber series. I think this may have been new ground for me—I know I petered out at some point when the books were originally published. Spoilers follow:
Anyway, Merlin has been imprisoned in a crystalline cavern by Luke, a longtime friend on Shadow Earth, only recently revealed to have ulterior motives, and hostile knowledge of Amber. Many adventures unfold, as Merlin manages to escape, thanks to a dimwitted duo happening by his jail. Things get complicated as Merlin travels through Shadow: old familiar places like Amber and Earth, but also new sites like the Keep of the Four Worlds, a handy crossroards that (unfortunately) is under siege, and controlled by unfriendly forces anyway.
A number of new characters (or are they new?), shifting alliances, and a usual display of spectacular magic. The climax finds Merlin (once again) seemingly trapped, but in a world that will be familiar to Lewis Carroll fans.