Leanpub doesn't have a box where you enter coupon codes. Instead, we have coupon links.
When an author makes a discount coupon, they do so by adding /c/ and some coupon code to the end of their Leanpub landing page URL.
Normally, an author will provide you with the full link. Here is an example of what this would look like:
If all the author provides you with is the code - in this case it would be the part at the end,
discount-code - just go to their leanpub landing page, and add
/c/discount-code to the URL, and you should see the coupon discount, if the coupon is still valid.
If someone has made a multiple-copy purchase to distribute copies to a group of individuals, they may have provided you with a "purchase token". If that is the case, please go here to redeem your token:
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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