Some Leanpub ebooks are not available in the MOBI format, which is required for the Send to Kindle feature. If you do not see the Send to Kindle button next to an ebook in your Leanpub library, that means the ebook is not available in the MOBI format.

Here is an article with information about how to know what formats a Leanpub book is available in:

http://help.leanpub.com/en/articles/2921549-what-formats-do-leanpub-books-come-in

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