To get a Leanpub book onto your Kindle, please follow the instructions on your Kindle Settings page.
Once you're set up, go to your Leanpub library, select the book, and click the "Send to Kindle" link you will find there. (If you don't see the "Send to Kindle" link, that means this book is not available in the MOBI format required by Kindle.)
Alternatively, you can download the MOBI file for the book onto your computer or device and use one of Amazon's apps to get a book onto your Kindle.
A book that is over 49MB in size cannot be sent to your Kindle from your Leanpub library. To get a book over 49MB onto your Kindle device, you should transfer it manually. To transfer a book manually, download the book file to your computer from your Leanpub library. Then, connect your Kindle to your computer via the Kindle's USB cable and manually copy or drag the MOBI file for the book from your computer into the "Documents" folder on your Kindle.