After the events of the War of the Ring at the start of the Fourth Age of Middle-earth, King Aragorn Elessar granted the Hobbits of the Shire effective self-rule inside his Reunited Kingdom, banning any Men from entering the land.
In FO 32 (SR 1452), King Aragorn also granted the Shire a stretch of new land: this reached from the ancient western borders of the Shire, the White Downs, to the Tower Hills. Thereafter, the area between the downs and the hills became known as the Westmarch.
The eldest daughter of Mayor Samwise Gamgee, Elanor the Fair, married Fastred of Greenholm, and they moved to the Westmarch settling in the town of Undertowers at the foot of the Tower Hills. After the passing of Master Samwise, they and their children became known as the Fairbairns of the Towers or Wardens of Westmarch, and the Red Book of Frodo and Bilbo Baggins passed into their keeping, becoming known as the Red Book of Westmarch.