Webin the wizard's secret workshop, defended by a locked grate and multiple traps, players can discover a book called the necromancy of thay.
Webdepends on what you want.
If you want astarion to like you and advance his quest a little faster, i suggest giving the book to him.
Keeping it to yourself adds some extra quest.
Webthe necromancy of thay has three main paths that can be chosen, each with different outcomes.
Giving the book to astarion.
Giving it to him will net approval from astarion.