We don't yet have Direct Messaging built into the platform exactly, but you CAN use the Transfer dialog with Memo in the Wallet, to send someone some STEEM, then include your email address to send a copy of the book to.

https://steemit.com/steem-help/@steempowerwhale/how-to-contact-anyone-on-steemit-directly-the-visual-quick-help-guide

I would recommend allowing folks to download it on torrents, rather than hosting it on a central server. You can give them a password for a zip file, for example, and a torrent magnet link.

I would actually suggest you publish the book for Upvotes and Pay What You Can donations in STEEM, and see if you don't do better than publishing to AMZ, etc.