This is dangerous for a couple reasons:
1. In general self-cursing is tough as a penalty, because once the Curses are gone, there is no penalty.
2. When the benefit is such massive drawing, it's often not going to be a problem to just trash the curses anyway.
So I'm not sure you'll be able to balance this card, but things you can think about are:
1. Make the benefit conditional on actually gaining the Curse
2. Put some sort of "while this is in play, any Curses you trash go to your discard pile (or deck top)".
Maybe something like this:
Forbidden Tome, $?, Action
Gain a Curse in your hand. Reveal any number of Curses from your hand. +2 Cards for each Curse revealed this way.
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While this is in play, any Curses you trash go on top your deck instead.
