Ok. It isn't really THE ancient book. Just some published pranks and imitations based in Lovecraft Mythos (a fictional universe based on the horror books from H.P. Lovecraft). And others are from original (and still imaginary) inspiration. But none of them was written before the 20th century.

If you want to see the consequences of reading forbidden books, watch the 80's Evil Dead trilogy (just be prepared for lots of senseless, and random gore, horror and comedy).