I used newsletter services like BookDoggy, Book Raid and Fussy Librarian. They are cheap, but their readerships usually gives you a good return.
However, I stopped using free Kindle deals and go with the countdown instead by discounting the books down to $0.99 which I then promote with FB ads. I found that a lot of bargain hunters just download any free book and often don’t read it. The same goes with free Bookfunnel promotions to build your mailing list. A friend of mine did a survey and 60% of her subscribers never even read one of her books.
Otherwise, announce free promotions to your own mailing list, on all the social media platforms etc. There are authors that offer newsletter swaps which works if you find one in the same genre, however, you have to send their books out in return and some shy away from the opportunity unless you already have a sizable list.
Marketing is hard work and with only one book out, the payoff is limited. FB and Amazon ads can turn quickly into a money pit, so watch your budget closely. Good luck