With Kindlegraph of course!

The technology is straightforward enough — readers sign in using Twitter, and select the book which they would like ‘signed’ (authors, or their publishers, have to opt in to have their books featured). The request is then forwarded to the author who — using Docusign‘s API — can write a short message and digitally sign it. The signed message is then forwarded to the reader’s Kindle, as a separate file.