In the back of your first book you can mention the second book is coming, add an excerpt, etc. so they know to watch for it. Once the book is released, add a link to the book's sales page in the back of the first one.
But the biggest thing is a mailing list. Use an email newsletter service provider so you don't have to worry about breaking spam laws--MailChimp, AWeber, and Constant Contact are some that provide free subscriptions for you for the first 2000 subscribers or so. (Double check that though.) Set that up on your website so people can opt in to your mailing list. Add a link in the back of your books so they can subscribe to get an email when the next book is released.
Amazon will also do things like suggest your book to those who downloaded the first one.
But no, there is no way to send an email (yourself) to everyone who got the first book. As well, people who download a free book don't always read it. You have to be out there!
I love connecting with and helping other AQCers outside this forum as well. You can find me all over the place!
If you are looking for more about writing, you may find my blog helpful, as well as my Twitter feed:
*The Helpful Writer *Twitter
If you are a parent, you might be interested in my ideas on growing happy, healthy kids who'll thrive in this ever changing world (includes crafts, activities, games, articles, and fun!):
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I write stuff (www.jeanoram.com)