Legendes and Antiquities of the Ramtops

Mentioned in Lords and Ladies, this book by Birdwhistle records the duel between Granny Weatherwax and Diamanda.

Since the duel occurs in the Century of the Fruitbat and the book is about antiquities, it is perhaps written in the far future (or the Discworld timeline is even more convoluted than Thief of Time would imply)