I'm not a Gmail expert but I think it uses the Google Drive function when you preview / read attachments. You have obviously clicked "save to drive" or something unintentionally when viewing those two attachments. This is very easy to do as Gmail's interface is mostly cryptic icons these days.
You seem to have jumped to a conclusion that Google are offering a free chapter of your book to all Google drive users. This is wrong, and actually quite paranoid.* Please try to relax...
People sneer at me for using hosted Exchange and proper mail clients, but this kind of thing is exactly why - to a certain extent I have control over what software I use at each end. The provider I use kept Exchange 2007 around for ages after 2010 was released, and I'm still on 2010 more than 2 years after 2013 was released. I can also pick whatever Exchange-compatible mail client I like, or use the web system.
Google, on the other hand, just changes things all the time, both interface and functionality. This is fine for some people but not for others. Personally I find it extremely annoying (guess I'm getting old
) and that's why I don't use any Google services any more (apart from search through DDG).
*Just because you're paranoid, it doesn't mean that they aren't all out to get you.