@Guy_V
I don't know if the comic will state it or not, as it doesn't go into details at times, and I certainly haven't read ahead on the comic to know. The novel on the other hand explains what happened, and it could very well appear next chapter. I don't think it is that important, but I'll still put it in a spoiler tag.
The Wisdom increase isn't from reading the book, but something like completing a sub-objective in his class quest. It is a flat increase of what I recall is +500 Wisdom. There is a book for each element, thus there are two purple books gathered (Fire / Poison).
So purple is something to do with the doors opened / specializations he had acquired. And gives wisdom. But he has to re-read if he already read them. Hmhm... I wonder if the contents of the book change once he opens the responding door? 🤔