You need to get book for Jess in LH2, and in LH3 you need to have 55 Arcana, 55 History and 55 Endurance to succesfully finish researches and ritual.
Variable soulburner_knowledge, required for this class, set as long as she was able to read the book.
@Looking_For_Cult I can advice you to try to look on games script itself, if you not afraid of spoilers.
