I have a feeling the book copy mechanic kinda messes with the balance of VBE. Or even shows that the balance could be improved upon? You decide if that's even a goal of yours, but here are my points:
As is:
* books give exp points based on their quality
* quality of a book determines its price (i think a legendary book is at 3500 silver?)
* colonists can read any book as long as they like and they get points until they reach level 20
* with this mod you can copy a book, which keeps its quality (right?) but decreases its value (right?)

My gripes with the balance as is:
* I don't think any book should be able to level a colonist from level 1 to 20
* copying a book should not decrease its price by much. A first edition is cool, but it should only account for 10% (maybe)

My proposal, inspired by Project Zomboid:
* Books should only be able to teach a colonist in a range of level. A book for levels 5-10 gives exp, but outside that range it either gives nothing or only a % of exp
* The quality of a book would determine how much exp is given and also which range it covers. For example a awful book only covers level 1-3, a poor book 1-5, normal 3-8, good, 3-13, and so on. Some overlap should be there.
* reduce the overall price of books while reducing skill gain of high tier books, so making copies can be a valid form of production.