The novel is divided into nineteen chapters, which alternate between the male protagonist’s (Will‘s) point-of-view, and the female protagonist’s(Nexus‘) perspective. In the final chapter, 19, the two re-converge in a cinematic climax.
Further, because of the large scope of the novel and to suggest some thematic connections, the chapters are organized into four major sections: Flood, Firmament, Forge, and Frost/Foundation.
So:
I. Flood Chapters 1-6
(set in Spring, also connotes childhood)
II. Firmament Chapters 7-12
(set in Summer, also connotes adolescence)
III. Forge Chapters 13-16
(set in Autumn/Fall, also connotes young adulthood)
IV. Frost/Foundation Chapters 17-19
(set in Winter, also connotes maturity)