Doctorow recently published a column in the Guardian on the benefits of free ebooks:
Releasing a book as a free download isn't newsworthy in and of itself. It was, once upon a time, especially when that book had the backing of a major publisher. Publishers are often characterised as being conservative about the net, so it was surprising when it happened. These days, many writers have convinced their publishers to dip their toes in the water on this, and it's simply not notable when it happens again.
Which is not to say that free downloads have no role when it comes to promotion, publicity and marketing. Their main effect is to magnify any good feeling your book has generated, by making it simple for people who love the book to get it under the nose of their social circle.