From the introduction:
Explore wondrous islands and sail across a sea of starlight floating in the Astral Plane! Inside this sourcebook, you’ll discover an expansive 5e-compatible campaign setting filled with character options, naval combat rules, and monsters from across the Silvery Seas.
This book presents the Aetherial Expanse, a wonderous setting in the Astral Plane where rival powers from prime material worlds maneuver around a tentative ceasefire to lay claim to its territory, while independent pirates rebuff these attempts at external domination.
The Aetherial Expanse setting was first introduced in Ghostfire Gaming’s Fables Season 2 adventure, Pirates of the Aetherial Expanse. The book you hold builds upon that original content, consolidating elements from the adventure series and augmenting it with a wealth of new material to craft your own swashbuckling adventures in this magical setting.