Philosophical Literature
Yes, there's a bit of a plot to this book. But it's pretty clear from the get-go that de Pizan wrote this thing to spread her ideas about womankind and to criticize all the horrible things men had written about women over the years. Sure, there are three different ladies who show up and tell Christine all kinds of useful things. But there's not really a conversation going on here.
Christine is speaking through each of the ladies, then turning back to her own character to say, "Oh gee, well I see now that you're totally right." It's not like Christine's character is debating any of the stuff the women say, though it's true that she asks some follow-up questions.
What she's doing is speaking directly to the reader, both as herself and through the filter of the Ladies Justice, Reason, and Rectitude. And she's lecturing us on her philosophical stance on women's value.
And just to drive it home, you know you're definitely looking at a book that falls within the philosophical literature genre when philosophical ideas are overwhelmingly more important than the plot or characters.