I love the Pandora Principle book, but the Spock/Saavik Pon Farr thing was canon and the "she's like a daughter to him" was a retcon for the book. In the original script for Episode IV, the reason that Saavik stays on Vulcan is because she's pregnant. So, much as I love Pandora Principle, I can't fault later writers for going with the on-screen relationship as their cue rather than the book (excellent as it is). And I like some of those other books, so. 🤷‍♀️