Flourens Delannoy was a linguist and a historian from Vicovaro who lived roughly 200 years after the conclusion of events in the Witcher series. He is the author of Fairy Tales and Stories (also translated as Tales and Legends) that features stories from and about the time of the witcher Geralt and his contemporaries.
Encyclopaedia Maxima Mundi entry
In The Witcher computer game
Delannoy is only mentioned in passing in The Witcher, as the author of the Fairytales and Stories. The book's inclusion and mention of its author is an anachronism since it was written 200 years after the period in which the game is set.