Killing her is incredibly easy- just get all of your physical damage dealers to flank her and knock her over with Battering Ram or Battle stomp. She doesn't have much physical armour so this is the most effective way i can think of. Rogues backlashing behind her and backstabbing her helps too

If you kill her at level 16, you will get about 17.5k gold from her corpse and 3 top tier skill books in summoning, necromancy and polymorph. And about 7k experience.

Donating the book with grant +95 attitude rather than 94 gold FYI

The maximum discount you can receive appears to be 52.4%, between bartering and attitude I got it up to a 71% discount but her prices where stuck at the 52% figure.

This quest doesn't bugged in Definitive edition. Just talk with her and don't say you like blackpits. Do all chatting and quit talking. Craft corpse explosion and quest log will updated. Select first choice(tell her you have just the book she's looking for) and in the trade menu just select corpse explosion and don't want anything. Quest will updated again and eithne's location marked on the map. Go to the location and quest will finished Note: If you give her corpse explosion book, you can't trade with her again so buy anything you like before this.

this quest is bugged in difinity edition doseent matter if donating or trading the book ...