Perks, not skills
Can't level up - i'm not sure if it only applies to that rebirth or if you can't have any progress on that book- I presume just that rebirth (So the first edition you finish that rebirth, if you haven't bought any perks with your level up points)
If you finish all books before understanding, it just gives it to you
Assuming you didn't spend any level up points at all, then possibly this means you have to start the book from scratch- like finishing an edition in the previous rebirth and then completing the whole entire thing, from the start, without using any level up points
Either way, the achievements do so little and it gives you it once you finish them all regardless