In the novels, Mance Rayder’s wife whose baby Jon swaps with Gilly’s son is Dalla.
Dalla is a wildling woman and the wife of Mance Rayder, the self-styled King-beyond-the-Wall. She gives birth to a son during the wildlings’ campaign against the Wall, but dies shortly afterward. Her sister Val survives and later becomes an important intermediary between Jon Snow and the Free Folk.
The baby-swap plot occurs after Gilly gives birth to a son at Craster’s Keep. Because Melisandre may seek to sacrifice Mance’s child for his royal blood, Jon arranges for Gilly’s baby and Mance’s baby to be exchanged. Gilly travels south with Mance’s son, publicly presenting him as her own, while her real son remains at the Wall.
This detail is often confused with the television version, which changes and streamlines the storyline. In the books, Dalla is the mother who dies in childbirth, while Gilly is the woman who protects the hidden royal infant.