From Digital Book World:
Book price promotions are one of the most useful strategies independent authors have for finding new readers and shifting units. Price promotions are like sales: authors lower their price of their book to encourage new people to try it. However, temporarily reducing your book to 99 cents (or free) won’t matter unless the right readers know about your price promotion.
That’s where online book promotion services step into the picture. If you’re not familiar with sites like BookBub, they’re places where readers can find discounted and free books — and sign up for newsletters to notify them of the latest deals within their genres of interest. Getting your book featured by the most popular services is incredibly valuable, will almost guarantee a bump in sales and downloads. However, these placements are not guaranteed, and your promotion can always benefit from being featured elsewhere. In these situations, smaller promo services are worth considering.
Reedsy has released an evolving directory of Book Promotion Services. Authors can use it to search for prospective book promotion services, sorting by genre, advertising costs, and mailing list size.
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Rhetorical question: when you’re planning your price promotions, would you rather put all your eggs in one basket or try to get every last bit of exposure possible?
Let’s say you’re planning a week free promotion with the aim of getting 50 reviews for your self-published book. You want to ensure that you’re getting a steady flow of downloads, as it will improve your rank in Amazon’s Free store much more than a one-day spike.
To “trickle-in” these new readers, you will arrange for your “Tier I” promotions (the ones with the greatest track record, with whom it is commonly more expensive to work) to run on different days, supported by promotions on Tier II, Tier III (and maybe Tier IV) sites. For example, the first three days of your Amazon Free Promotion might be supported by paid promotional placements as follows: