Publishing on Google Play

I'm in the midst of publishing on Google Play. I found it slightly confusing at first, until I found their Beginner's Guide. When I started signing up for the Google Books Partner Program, I thought I was in the wrong place; Google Books is for making book content searchable, but I want to sell my book, not make its content freely available. Then I discovered that you can limit access by a given user to 20% of your book's content, and that you have to participate in this search/preview program in order to sell through Google Play. This is really just their way of providing something similar to Amazon's "Look Inside!" feature.