I'm trying to build my mailing list to 10,000 subscribers, preferably most of them organic. I have links to the email list subscribe page on my blog at the front and back of all of my books, with a 4-book bundle for a newsletter incentive in the backmatter (basically, a four book sampler containing two permafrees, a short story, and a book I wrote on my blog).
I've been on Prolific Works for several years now, and gotten many subscribers from that, but I'd prefer to get organic signups if at all possible (though I suppose, if all of your giveaways are opt-in, that could be considered "organic"). Lately, I've found that running featured deals on sites/newsletters like BookBub, Freebooksy, Book Barbarian, etc all increase the rate at which readers sign up for my email list, but not as much as I would like.
What techniques have you guys found to help increase organic newsletter signups?