Turn the sentence from active voice into passive voice:
They have already discussed the book.

The book has already been discussed by them.

Hope this helps!

boccaccio employs the voice of his narrator dioneo to represent his ideas on the tension between masculinity and femininity.  while dioneo excites the members of the brigata and recognizes women for their unique skills and independent ideas, he concludes the decameron with a story reminiscent of the society from which these young aristocrats escaped, revealing misogynistic overtones.  dioneo boccaccio is able to explore the gender disparity which is inherent in 14th century florentine society.