You can integrate Bookfunnel with Mailerlite (and probably others). So, if you put a reader magnet in the front of your books, and people go to Bookfunnel to grab it, you can insist (or ask, either way) that they sign up to your mailing list. You can tag those subscribers so you know where they came from, too.
The shared promos are part of it, but for example if your forum sig here said 'Get my first book free' with a bookfunnel link, people could go to that link and decide whether to join your mailing list or not. Some join, grab the book and immediately leave, but that's normal. Anyway, if they enjoy the book they'll most likely stick around.