Speaking to French fantasy magazine Chimères, Andrzej Sapkowski said that, though he has no plans for what comes after it, he has completed another Witcher book. "I have finished the book," he said, "It took me maybe two years—something like this."
While the saga told in Sapkowski's Witcher books reached their conclusion with The Lady of the Lake, published in 1999, he returned to Geralt's adventures with Season of Storms in 2013, most of which is set between the events of the early short stories. Presumably this book is also set somewhere within the existing timeline since Sapkowski previously said he wasn't interested in extending it beyond that.
"The story is complete," he said at Warsaw Comic Con in 2018. "The saga has been concluded, so if by any chance I write something in The Witcher universe, and I sure have such intention, it would probably be something like a prequel or a sidequel. Not a sequel."
The book is scheduled to be published in Polish by the end of the year, with an English translation due in early 2025. Meanwhile, The Witcher 4 remains in development at CD Projekt Red, with voice actor Doug Cockle recently confirming that he'll be back to voice Geralt, though he won't be the main character this time.