How to search books by author name?

Question

I'm using the Amazon Product Advertising Ruby API (amazon-ecs) library. I have a list of some "vips" and I need to know if they are also the book's writer. So I need to know how can I do a ItemSearch by author name.

$response = $amazonEcs->returnType(1)
                  ->responseGroup('Large')
                  ->optionalParameters(array('ItemPage' => 1))
                  ->category('Books')
                  ->page(1)
                  ->author($author_name);

How would I accomplish this?