To contact a Leanpub author, please go to the web page on Leanpub where you found their book. Then, scroll down and click the "Email the Author(s)" link, which will look like this:
If you don't see that link, you can try contacting the author some other way. Many Leanpub authors share their email address near the beginning of their book, and also share their social media links on their Leanpub profile pages.
Typically Leanpub authors welcome questions and feedback about their work! It helps them understand what people are thinking, and often helps them improve their books or courses for everyone.