As of 2015, the only official sequel to a Nicolas Sparks novel is the 2005 novel "At First Sight," the sequel to "True Believer." However, the 2003 novel "The Wedding" also functions as an unofficial sequel to 1996's "The Notebook," since it is about the same family.
"At First Sight" is a direct sequel to "True Believer," following the same characters and picking up shortly after the first novel ends. "The Wedding," on the other hand, follows the daughter of the main characters of "The Notebook." Though Noah from "The Notebook" has a small role in "The Wedding," most of the events of the book are distanced from those of the previous novel.