I am wondering how to change the default order of weight, title for Book in Drupal 6.

I have a set of nodes displayed inside a page that use a estructured book. The problem is that i want to display them ordered by weight and random, instead of weight and name.

The book it's used to structure different types of local companies and restaurants.

I have traced the code looking for a solution, but it's a bit messy for me.

I don't know if i can override some hook in order to achieve this, so i came here because i'm near to throw the towel.