This is what I was told by another quilt book author. My understanding from what she told me is the designer and author of the book gets a lump sum by the publisher to write the book and then a % of each book sold. Even if the publisher sold thousands at a lower price to discount buyer, the author still gets her % of the books sold. They don't lose money when their book is sold at discount by the publisher.
They can buy their own books at discount, get the % of those books as profit and sell the books at MSRP and get more profit.