To add a Thank You note for buyers of your book, go here:
...making sure to replace "YOUR_BOOK" with your book's unique address.
You can write separate messages, one for the web page the reader is taken to after they buy your book, and one for the email that they will receive.
Here is an example of simple Thank You messages you can add:
Here is what this will look like on the post-purchase page, where customers go after they make a Leanpub purchase (the message appears underneath the book):
Here is what this will look like in the purchase email (the message appears above the book):