The Liber Marium

(attempt 4) (I see this is the favorite!)

In the Fourteen Deserts, water is a valuable and scarce resource outside of oases. However, those who are lucky and well-connected might be able to find one of these tomes in a black market. The Liber Marium is a grimoire (a book of magic) from another world on the western edge of the Pit of Hell, whereas the Deserts are on the eastern edge. This world is a large ocean with nineteen islands. The Liber Marium is a compendium of various spells and rituals from that world that involve the manipulation and spontaneous generation of water, communication with aquatic life, wind manipulation, and various other curious things. In the Deserts, those who are fortunate enough to find one of these rare tomes often use them to establish new communities by creating springs and lakes artificially, and travelers may make use of various spells to generate clean water wherever they may be.