While I wouldn't put it besides CD Projekt to pull some scummy moves, from what I'm reading they seem to be in the right here. Even the author himself seems to be aware of that:
“I was stupid enough to sell them rights to the whole bunch,” Sapkowski said at the time. “They offered me a percentage of their profits. I said, ‘No, there will be no profit at all — give me all my money right now! The whole amount.’ It was stupid. I was stupid enough to leave everything in their hands because I didn’t believe in their success. But who could foresee their success? I couldn’t.”
Yeah, so you made a dumb choice and now you're suing the people who turned your stories into a worldwide success for quick cash. That's low, man. Really, really low. If anything, he should be thankful to CD Projekt. I'm pretty sure his Witcher books sell considerably more since the games came out. I'm all for rooting for the underdog, but this guy comes across as a bit of a douche.