Riker decided Data was subhuman scum.
Remember, Carey wrote this book so early in the process that all she had to go on were the writers' bible and the "Farpoint" script. And in the original writers' bible, "Bill" Riker's prejudice toward Data was presented as a significant facet of his character, something that would be an ongoing source of conflict between the two of them. Carey couldn't know at the time that it would never be mentioned again after "Farpoint."
All of the first original novels in the post-TOS series -- Ghost Ship, The Siege, The Escape
, and By the Book
-- were written by authors who had only series bibles and early scripts to work from. I think the reason Ghost Ship
feels the most "off" in retrospect is because TNG was the series that underwent the most evolution and transformation in its early days, that diverged the most from what was written in the bible. But that's hardly Carey's fault. She did the best she could with the resources she had.