I've a Kindle so Amazon is super-convenient, however not always cheapest. I came across one store selling a book I wanted in other formats (including PDF) quite a bit cheaper, but that site was very lacking in a decent selection.

For those using readers by Sony, etc, are there any very good online stores or do you have to trawl the web any time you want a book?

Note: I'm only interested in legal ways to get books, i.e. not sites selling/giving away ripped copies.

Having to know the publisher immediately means my search for books is fragmented, which is exactly the problem I want to solve. I wondered if there is anywhere the equivalent of Amazon, selling a massive range of books from multiple publishers in PDF or other format.