The short answer here is that there is only a free sample available for a book if the author has chosen to create one. So, some books and have a free sample available, and others do not.
To see if there is a sample available, go to the landing page for the book and look for the sample button under the title, like in this example:
OK, that's it!
If you have any feedback or questions about this article, please email the Leanpub team about it at firstname.lastname@example.org!
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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