The Prairie Legacy series is a continuation of the Love Comes Softly series. It follows the life of Clark and Marty's youngest daughter Belinda. This may be confusing to those who have only watched the movies because Belinda is already in the story but the writers changed the story to fit their wants and needs, I suppose.
The serires is great but you may want to read the Love Comes Softly books before you get into these. They were all written in the Little House era and are wonderful, much better than the films in my opinion.