Well, you'd have to purchase the electronic version of Twilight, but yeah, that's pretty much the whole point of the Kindle.
But no, not everything is available in electronic format.
Why isn't everything?
See my post. To elaborate a little bit, when an author sells their publishing rights, electronic rights may or may not be included in the agreement. In order to sell an electronic copy of a book, Amazon must have licensed it meaning the publisher and author have to have worked out a digital distribution agreement.
For physical books there are no such complications.