I implemented something like this once. The trick is embedding the digital signature into an ebook file format that was not designed to accept it. For instance,

I implemented something like this once. The trick is embedding the digital signature into an ebook file format that was not designed to accept it. For instance, the following method is secure, but the result doesn't look like an ebook: 1. I email the author my public key. 2. The author hashes a copy of the ebook, signs the hash with his/her private key, and puts the resulting PKCS #7 file on a web server or emails it back to me. What is needed is an extensible ebook format (or a permissive one like JPEG that lets you append arbitrary data to the file without corrupting it).