At Leanpub, we actively encourage readers and authors to interact: it's all part of the Lean Publishing process.
You can usually contact an author using the "Email the Author(s)" form on their book's landing page:
Also, many Leanpub authors include an email address in a section near the beginning of their book, where they ask for feedback.
We also let authors set up an online forum for each book. If a book has an activated online forum, you can see a link to the forum, if you select the book in your Leanpub library. Here's a screenshot of what this looks like:
Many Leanpub authors will also let you know their Twitter, Instagram, or GitHub account, which can find under their picture, at the top of their author profile page on Leanpub.