"Saltwater Witch is the "prequel" to the other two Seaborn books, five years before Seaborn (somewhat YA). It's actually standalone, and the second and third books in the series, Seaborn and Sea Throne, can be read as one book.
Saltwater Witch follows Kassandra through a sort of self-discovery, trying to find out how she got to Nebraska when she clearly doesn't belong there."
