By default, we have a comment form (using Disqus) linked from the book landing
page, and an "Email the Author(s)" feature readers can use to send you emails (this will not reveal your email address to them).
The best way to get feedback from readers is to put your email address in your book and ask readers to email you. Many of our successful authors do that.
Other options include creating your own Google Group or a Slack channel.