@Jeto: I have thought a lot about the changes to the spellbook and I would like to ask if you could take another suggestion to the team:
Would it be possible to bind the spellbook (and thus the spellbook level) to the class instead of the character?
Why does this matter?
Currently, the only reason to actively use multiple characters of the same class in PQ3 is the difference in GFX / SFX (male / female, cat / lizard, …) but nothing else. With the proposed change that the spellbook is bound to the character and not the class, players will be set back when trying out the other GFX/SFX model.
Will this unfair / provide unfair advantages?
I don’t think that this will create any advantage considering end-game: you do not need the 2nd character, there are no additional bonus rewards (only first-time reward is relevant) and the gender/model does not have any impact on the playstyle (it is purely cosmetic).
On the other hand, focusing on the cosmetic aspect, being able to use the second class-model may motivate people to invest into skins, as they may look quite well on a different model (for example, a skin used for the female merc might not look good on the lizard or vice versa). 
TL;DR: Please do consider changing this, as it does not yield any advantage but has a huge impact for people caring about cosmetics / model GFX and skins
Thanks in advance!