Leanpub lets you add Google Analytics code to your landing page, so you can use Google Analytics to track sources of traffic to your page.
After you create your book, in your Book Tools menu, select "Selling & Royalties" and then click on "Analytics". This where you can enter your Google Analytics code.
One of our authors, Visnja Zeljeznjak, has written an amazing article for our blog on this subject. If you're looking for a more detailed rundown about setting up your book with Google Analytics, you should check out this blog post.
Please note you should also read this update, which has some information that has changed since Visnja's post: https://leanpub.com/blog/2015/09/google-analytics-update.html.