I have bought books from Amazon and Manning.com, mostly technical books. I have to say, I am so happy with the business model of Manning.com:

- they offer good quality books

- they also offer a subscription where you can view all books and download one book per month

- when you buy a digital book, they give you the book in several formats (kindle and pdf)

- you can read the ebook in multiple devices

O'Reilly should follow the same business model.