There are different ways to support players with skill books. You already are able to find some skill book all over the place. But current most of them are pretty obsolete because you probably already bought the skills from a merchant. You could also give skill books as quest rewards or make a quest giver teach you one skill of your choice (or one of your teammates).
And why do you need to lower the level? You just need to ajust their stats/armour and they skillset. At the moment it's one of the issues, that pretty every enemies fights you with the same skills as all the others?
The first act needs a rebalance anyway, because it at current it is not balanced at all and till the end game is fixed with all mechanics it won't be balancable anyway.
