"Knight of Shadows" is Merlin's Vision Quest, similar to Corwin's allegorical journey in the last book of the Amber series. Although I still like the two books, Zelazny's normal fascinating stories slow down considerably when he decides to go this route. Merlin becomes trapped in a symbolic battle between the Pattern and the Logrus, and for most of the book we have no idea where Merlin's encounter with a land between shadows relates to the three plus books leading up to here.
On the good side, things liven up quite a bit towards the end of Merlin's trials here. The magic and Merlin's pluckiness get fun again.
Zelazny introduces a new type of character into the series in "Knight of Shadows". They become surprisingly interesting and sympathetic figures in the story, and I thought they were a very imaginative idea. I won't give more details about them, because that would spoil the fun. :-)
As in all the Amber books, the volume cannot stand on its own. It is in essence the fourth 20% of one very long book, since Merlin's story starts on page one of book one and doesn't slow down until the last page of book five ... if you can even consider that to be the end. LOL
I consider both five book series to be highly original and imaginative sets of fiction, and have always applauded Roger Zelazny for the hours of entertainment he thereby provided me.