How could you not realize a book that was giving you +1 skill point over and over and over again wasn't bugged?
This is how it happens:
You end up finding the keeper of secretes around lvl 10-11, because you hoarded all those blood stones, hoping the game gave you an opportunity to heal someone. Alas so many souls were were doomed, even though you held the cure in your hand, by a lack of dialog options...
So at lvl 10-11 you find the skill books, and buy them of course. When you lvl up you find another pair there, and you assume the the game is balanced around getting a set of skill books every lvl up. Then you think, o'dang I kind of gimped my character by holding on to all those blood stones so long. But you push through.
Then as you continually check the vendor, you start to realize you are getting skill books more than just every level. You're not sure what's causing the vendor to reset, but you just realized you have an infinite source of skill books (as well as gold, legendaries, essences). You're first thought might be relief because you didn't gimp your character after all, but then you think how is all this balanced, I've got an infinite source of skill and stat points.
So you check forums to see how many skill books you are suppose to have, and realize ... hey you're only suppose to have one set, and you've essentially used a stat trainer unintentially all this time to overpower your character.
