Will's mother is mentioned in the first book, The Ruins of Gorlan, but her name is not mentioned. She was the wife of Daniel. The Ranger Halt was begged by a dying Daniel, who had been mortally wounded by Wargals, to look after Will's mother; she was alone and expecting a baby back at Daniel's farm. Halt tells Will in the first book that he helped her give birth, but it was a difficult birth and she died shortly after Will was born.
Later in The Death of a Hero, the true story of Will's mother was revealed to show she was actually killed by Kord and Jerrel trying to help Halt, and that she and her husband originally came from Norgate Fief. After her death, Halt buried her on the farm. Not knowing her name, he inscribed a stone with the words "A Brave Mother", and laid it on her grave. Halt claimed she died in childbirth because he was afraid that Will would hate him for causing the death of both of his parents.
The Ruins Of Gorlan (first mentioned)
The Lost Stories - Death of a Hero (first appearance)