Most Leanpub authors let you download a sample PDF for free from the book landing page, so you can see if you like what they're doing before you commit to buying their work.
If a sample version of a book is available, you'll see a "Read Free Sample" button on the book's landing page, like this:
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Interested in creating your first Leanpub book? Here are some quick tutorials for our most popular writing modes: http://help.leanpub.com/en/articles/3088382-quick-walkthroughs-for-getting-started-on-a-leanpub-book