First, note that there are two different meanings of "preview":
The "for the author only" version, which is what we call the preview. These are private to you, and not available to search engines.
The "make an HTML version of the book for search engines and readers who are interested in learning more, but which can be different from the book". These are for interested readers and for search engines, and are indexed by Google and other search engines. Note that the way that you make this is that you publish (not preview), and choose to generate the a sample version of the book, or a full version of the book, as HTML in your settings first.
To choose to make the full version of your book available as HTML when you publish, go to the Generations Settings page for your book.
You can choose to make either:
no web page
just a web page out of the Sample (which is defined by Sample.txt)
a web page out of the entire book
Obviously this last option makes the most content available in Google, which means you should get more search engine love, but at the cost of there being no "paying readers only" content.
For an example of what this looks like, see https://leanpub.com/markua/read ... This is what you should find as the top result if you Google for "markua spec".
Finally, note that when someone buys your book they can read it as PDF, EPUB, MOBI or on Leanpub (assuming you generated it using Leanpub). The "on Leanpub" thing is basically a "read the EPUB in the browser" feature, which is different than the HTML version that we generate above. It uses "pages" and pretends to look like an ebook, instead of the sample HTML version which is one big huge scrolling page...