There is currently no other way to get the ritualist book in the game except from the boss in the necro dungeon with a very low chance.
I think this is a very bad direction to implement new content for the game.
Ritualist content doesn't end when you get the book. Only then will it begin.
It's very similar to alchemy, you'll have to experiment a lot to make these monsters usefull. The only difference is that alchemy is not closed like this.
I don't feel like it should be so privileged.

Save/Change the Mortal Online 2 this time.

the decision is in your hands.