Well, I think so, I never heard about it before, but since I'm spending some time in Poland I see it everywhere, so I guess it has to be the newest. But how much it is related to the "main saga" then, I wonder?
Season of Storms is indeed the newest book about the Witcher by Andrjez Sapkowski. It is a standalone book, that does not mess with facts that happened in the Saga or the short stories.
Also, Sezon Burz takes place before the events of the books in the Saga. Around the same time of the short stories told in the Last Wish.