If you have a book that is established, Bookbub sends out emails to readers from across the board to download your work, but only if it is deeply discounted or free.
This is the second time I've used Bookbub to promote one of my books - and it ain't cheap, folks - but I have been very happy with the results. Bookbub probably works best if you have a series, which I do. Two of them, in fact. I have offered the first book of each, The Persephone Cole Vintage Mysteries and the Alvarez Family Murder Mysteries, for free. The hope is that if the reader likes the first free book, they will go and buy the others at normal cost.
I've heard of other authors who participate monthly in the free giveaways and have had great success. Some offer their works for free or 99 cents, both for a limited time. I have joined the group. I don't know if I can do it monthly - have I mentioned this ain't cheap? - especially if you do a free mystery. That category is at the top of the pay scale, coming in at my yearly shoe allowance.
With Amazon's Kindle now offering Kindle Unlimited as an option for its readers, I have no idea how it will change the Amazon/Bookbub marriage. Kindle Unlimited, copying Netflix's monthly charge for unlimited DVDs, is now offering free, unlimited book reads for a similar monthly charge. Will that affect the free book promotions? I have no idea. But I'm in it, folks, I'm in it. And I feel like I'm sitting at a blackjack table in Vegas. Winner takes it all. Loser selling small.