Delorna's Spellbook

Delorna's Spellbook is one of the three items required to build a Halruaan skyship.

Alatos and the Thieves' Guild
Alatos, the leader of the Baldur's Gate, has been contacted by the Halruaan mage, Resar, who wants the former to arrange the theft of three artifacts used to create skyships. Resar does not wish to see the secret of their creation spread beyond the borders of Halruaa, and Alatos has concluded that Gorion's Ward and their party would be the best choice for the job.

In-game description
"This item is a spell component required to create a Halruaan skyship."