You're going to need a Twitter account (and a participating author), but now you can.
Simply open the Kindle ebook (it has to be one Amazon sold you), highlight a passage and type @author and your question. If you're following the author on kindle.amazon.com, the question and the author’s answer will both become Public Notes in your book. The question and answer will also be visible on the author's page on kindle.amazon.com.
If your question is answered, you'll get an email from Amazon with the answer included. And even if the author doesn't answer, other readers can do so from the author page.
Amazon has 16 participating authors right now, and you can find a complete list as well as a FAQ.
You may not know this, but the Kindle has had similar @search options for some time, including @dict, @help, @store, @ur, @web, and @wiki. This new @author is a clever addition to the bunch. It's also a smart way to start tying the Kindle hardware with the Kindle Social Network.
BTW, Amazon also secured the @author account on Twitter, but I should warn you that it can't be used to ask a question.