Unless a Leanpub book is licensed with an appropriate Creative Commons license, third-party print-on-demand services like Lulu can only be used by Leanpub authors to produce printed copies of their books.
Leanpub readers are not allowed to upload Leanpub books to services like Lulu for producing their own print copies, since that would violate the author's copyright.
In terms of producing a print copy of an ebook for your own purposes, we recommend printing one chapter at a time, and either staple it, put it in a binder, or take it to a place like Staples or Kinkos to have it spiral bound. It is permitted for Leanpub readers to print their PDFs for personal use 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