"The Ashes of Eden" and "The Return" were both based on Shatner movie pitches, IIRC.
The Return definitely was, though I'm not sure about The Ashes of Eden.
"The Ashes of Eden" was apparently Shatner's proposed storyline for "Star Trek VI". After V bombed, I saw George Takei at a convention where he spoke about Shatner's proposal for the next movie, which would have Kirk leave Starfleet to defend a mysterious alien planet and the Enterprise crew would have to go after him.
When I read the book, I thought, "Hey, this sounds like Shatner's pitch that Takei spoke about."
Or this ...

"The Ashes of Eden" and "The Return" were both based on Shatner movie pitches, IIRC.
The Return definitely was, though I'm not sure about The Ashes of Eden.
The planet with a Fountain of Youth. Shatner wanted that for his pitch for ST VI (contractually, he was at last allowed to pitch a sequel to ST V - and direct it - because of his "Favored Nations" contract, IIRC), but was overruled on th story, and so he shunted into his first Trek novel, as to why his romantic interest, Teilani of Chal, had unusual restorative powers.
Ah, yes:
Wiki says: "Chal apparently has fountain of youth properties, which seem proven when Teilani's wound miraculously heals, and the anarchists want to sell it. Kirk accepts Teilani's offer to help protect Chal, seeing it as a second chance.."
"The Return" was from his pitch for "Generations II".