Josh called Nine Princes
Weakest of the Amber books?
Least Amberish, though.
TVtropes has a page on this phenomenon. They call it Early Installment Weirdness. And the simplest explanation is almost certainly the correct one. Zelazny made up Amber as he went along, discarding elements that he didn't like or didn't work, codifying what was Amber and what wasn't as the series progressed.
I had started a post on the thematic differences between 9P and the rest of the books, but never completed it, as it seemed like too much of a niche project, even for a site like this. There is more of an emphasis on probability (Flora, in reference to Carmella: "I have decided that it is improbable that she will answer the door") that never reappears, Rebma never plays a large role again, we never see another creature created out of the Shadows like Morgenstern. (Merlin finds a comparable horse and his comments imply Corwin's Star and Benedict's horse, whose name escapes me (Glendenning?) are creatures of a similar nature.) It's unclear if Julian literally created Morgenstern out of shadow and we never see such an act of creation throughout the rest of the series, or if Corwin was describing the act of finding him in shadow with very unusual language.