A book in Manannan's study. This is likely a magic book also related to studying the High Kingdom (see High and Low Kingdoms).
Behind the scenes[]
The 9th century Sanas Cormaic (Cormac's Glossary) euhemerizes Manannán as "a famous merchant" of the Isle of Man and the best sailor in western Europe, who knew by "studying the heavens" when the weather would be good and bad.