first off I don't think your spamming anyone, they ask for you to tweet it once to access the ebook, it's a pretty interesting method to read and it is free. The concept is cool and it makes sense
Thanks for clearing that up if there was any misunderstanding, it's part of the concept really, paying with a tweet gets Bitcoin more exposure through your twitter followers, the more people that click the link, the more tweets, the more tweets, the more people that continue clicking.. creating a viral effect