Audiobooks discussion

Sorry if this is not the right place for this question. I spend most of my time in this group so thought I would throw it out here.
I've been noticing that some books now have a green "read book" button just below the "add to my books" button. What is this/how different from add-to-my-books? Why only some books have this? (or maybe they all do now and I've only noticed it sporadically)

Goodreads has some ebooks that you can either read online, download a pdf copy, read or download excerpts.
You will only see those tabs on books that offer one of those options. There is a complete list of all available books under the "explore" tab in the goodreads header at the top of the page ("ebooks).

Great - thanks for the info!
In particular, I didn't know about the "explore>ebooks" item.