火的考验 (Trial by Fire)

This can be done in an ultimate good playthrough.

If you do enough good deeds to trigger the quests from the guard in Taskski and complete these, then you can talk to Tancred in Darsov to trigger the attack on the cult. He will lead you through attacking a number of cult hideouts and eventually their stronghold. Once everyone in the cult is dead you can claim the book from the dead leader.