on BookBaby:

Enlisting a book discovery service promoting cheap or free eBooks to a subscriber list is one way to get reviews, grow your social network, find readers, and increase book sales.

Whatever your long-term plans for promoting your book, free or bargain book listings could be one way to kick-start your promotion or energize back-catalog titles for a new book release. As with any book promotion, it’s up to you to vet providers and make the right decisions regarding where to spend your money and how to position yourself in the market. In other words, we’re not specifically recommending this approach, but if you’re inclined to give this promotion tactic a go to aid in book discovery and sales, here’s a list to set you on your way.

The idea is simple enough: These services give you access to their lists — via email, websites, and social media — for a fee. Most are up front with their subscribers that these promotions are advertorials, and each has its set of criteria. Some services boast stringent editorial review, meaning every title is approved by an editorial staff before being included, while others guarantee placement. This means that, should your book not make the cut at one service, others will promote your title.

Here are 13 book discovery services that cater to independent authors. An asterisk designates a “Friend of BookBaby” as listed on the BookBaby website.