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.

O'reilly, Packt Pub and Sitepoint all have ebooks for sale straight from the publisher

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.

Depending on what kind of reading you enjoy, you might find some good books at E23. E23 is the e-store for the makers of the RPG called GURPS, but they've signed up many other producers of RPG materials. I really love their download system (unlimited downloads if you register, no DRM, etc) and feel it really treats me like the paying customer that I am.

I don't know how well the kindle will handle RPG pdfs, though, as the tables can be quite complicated.