If you’re looking for feedback, the best thing to do is to put a section at the very beginning of your book where you ask for feedback, and give people your email address. Many Leanpub authors do this, and it works really well.
On all Leanpub book landing pages, there is an “Email the Author(s)” button that people can click, to email you. You can also mention this in your book, and in the About the Book section on your book’s landing page.
If you’re familiar with GitHub, another thing to do is to set up a public repo on GitHub with an issue tracker, so people can report issues to you about the book. You can point people to this public repo in a section at the beginning of your book.
When you go to publish a new version of your book, on that page there is an option to email your readers with a note that you have updated the book. This is useful when you have published a significant amount of new content, like a whole new chapter, but it should not be used to spam readers, because that makes them unhappy. If you’re publishing your book in-progress, the next time you publish a new chapter, you could use this feature to explain the new content, and to also ask for feedback, and give people your email address.
When someone buys your book, they also have an option to opt in to share their email address with you. It is ok to email these people directly. You can find the list of people who have opted in to share their email address with you here: