It feels like this more like an artifact for some of our companions (who can also be played as protagonists in the final version), not the custom MC. For example, to Gale the book is just another magic slurpy, but for Astarion it's so important that there are upcoming cutscenes of him interacting with it and trying to learn its secrets. At first I thought "ah, good old Astarion being the power-hungry lil sucker". However, after a second run I suspect that he is hoping to find a way to un-undead himself (after all, the wizard who owned the place was hellbent on achieving true resurrection, not just making zombies) - and the book plays a major role in his playthrough.
