Currently, there is no way to automatically give each reader a unique coupon code.

However, if you would like to interact with individual customers who buy your book, in order to reward them with a custom coupon code of your own, there are a couple of options.

First, in your About the Book section, you can ask them to opt in to choose their email address with you, when they buy your book. You can then see their email address here:

Author > Books > [Your Book] > Community > Customers

Once you've confirmed their email address there, you can send them your unique coupon code.

Second, everybody who buys a Leanpub book gets a receipt with a unique Purchase ID. You can see the IDs for purchases of your book here: https://leanpub.com/YOURBOOK/individual_purchases. The path in the "breadcrumb" menu is:

Author > Books > [Your Book] > Store > Sales and Royalties

...and then click the "purchases" link at the very bottom of the page.
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So, someone who buys a book could share their Purchase ID with you, and you could check it against your records, to see if it's real.

If a customer can't find their receipt email, they can sign in to Leanpub and go to https://leanpub.com/user_dashboard/purchases, select the purchase, and send themselves a new receipt email.