These are currently the biggest thing keeping players from progressing through their professions. They take fat too long to get. Currently, running solo fortresses at rank 11 Professor takes over an hour, just to earn 2 Spell Books. I takes 1k challenge exp at rank 49 for 2 Books. You are basically forcing people to sit at fortresses for hours on end, just to make minor progress to getting their next skill.
You allow players to change professions at will, but you are forcing people to stick to one profession, in order to be proficient at Challenges. Without the ability to earn enough to boost a secondary profession, you are making it harder to form groups, since most people are going to choose Auror or Professor, due to not being able to consistently find groups to battle fortresses and Magizoologist having too low of damage to be a viable solo profession.
Restricted Books force people to choose one profession as their main, so why limit the skill books and take away the ability to have a secondary profession?
If you so make it easier to obtain skill books, you will need to retroactively award books that would have been earned under any new system as well.