-Get a mailing list and advertise it at the end of your books. (A lot of people use mailchimp.)
Yes, lots of people said they can't wait for the next book, but without a mailing list you effectively have no way of letting them know that the book is now available. Sure, you can email the ones who emailed you directly, but I'm sure that was just a small fraction of those who enjoyed the book and would read the next. So unless you have their email address or they liked you on Facebook, you really have no way of reaching them.
Put a Mailchimp signup on your website, mention it in your bio on Amazon (tell them to visit your website to sign up for news on new releases), mention in the product description on Amazon/B&N/Kobo/iTunes, etc, and most importantly, put it at the end of EACH book.
You can't go back in time and get all those people you missed, but you can do a better job going forward.
(I speak from experience...I'd sold more than 12,000 copies before I finally broke down and got a mailing list, and now I basically have to start from scratch. I was stupid. Don't be stupid.)