One of the things I really like about eReaders is the built in dictionary. Unfortunately, they aren't always comprehensive and, of course, fail when an author is making up new words.
So I came up with an improvement: eBooks should bundle a book-specific dictionary that can be merged with the standard dictionary while reading that book. This allows books to provide specific definitions of uncommon words and new words.
I'm re-reading The Lord of the Rings right now and Tolkien uses a lot of obscure and archaic words that are real (but which my eReader's dictionary doesn't know) and he also has a million made up words in Elvish, Dwarvish, Entish, "common tongue," etc. It would be helpful to be able to look those up as well.
So...Amazon, go make it happen.