Magnitus: My ereader allows for the writing of notes in a book so assuming that the book is DRM-free, you could get the author to sign it.
This is for select ereaders and select books though.
tinyE: While I don't really have an interest in that I am none the less intrigued. I wonder if a actual autographed Vonnegut would fetch the same price as an ereader autographed Vonnegut. Someone out there must know this stuff.
Well, it would definitely be easier to forge so unless they integrated strong certificate based encryption to authenticate the signature, I'd say no.
I don't use that feature much myself. I think it was mostly meant for students reading textbooks.
While I do use it to read technical books, I am not directly tested based on my readings so I don't feel the need to annotate them, though perhaps it would be a good practice.