The license key is generated by Gumroad and it looks like this:
7BDE50F1-FE6545E6-A8FD5AD4-A7A300B7. After you bought the book you can find your key in several places:
It is included in the confirmation email you got when you bought the book.
In your Gumroad account, find it in the Library/Product's download page.
Check it here: What is my license key?
Contact me directly at firstname.lastname@example.org.