To merge accounts, please sign in to the account you don't want to keep, and go here:
On that page, you can transfer your purchases from that Leanpub account, to your other Leanpub account.
After you transfer your purchases to the account you do want to keep, you can close the account you don't want to keep here:
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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