Self-published authors should consider offering their books on subscription services because, as far as publishing goes, the more venues the better. You should - after perhaps an initial foray into Amazon KDP Select - get your book onto as many sites, and in as many formats, as you can.
Of the three subscription services listed below, Scribd and Oyster are the least restrictive. (Amazon demands exclusivity.) I would recommend going through Smashwords to get your books onto these services because 1) you will get a royalty and 2) Smashwords also makes your book available to libraries.
Scribd is a subscription service with 80 million unique visitors a month. For a monthly fee of $8.99 subscribers can read unlimited books. The Scribd library boasts more than half a million ebooks and over 30,000 audiobooks, including New York Times bestsellers, Pulitzer Prize winners, as well as self-published works. 60+ million documents have been uploaded by users, Essentially, anyone can upload a file onto Scribd, which has led to several copyright infringements, Although Scribd has a huge readership, the best way to ensure that they notice your book, and that you get some monetary compensation, is to go through your publisher. As a self-publisher, you can use Smashwords or BookBaby, each of which offers a Scribd royalty payment.
Recently, Scribd secured $22 million in financing led by Khosla Ventures and including new funds from existing investors. The new funding brings Scribd’s total financing to date to $48 million.
Note: Oyster announced in September 2015 that it would be shutting down its service.
Oyster was launched in 2013 and is headquartered in New York. With half a million titles and 1,600 publishing partners, Oyster claims to be the leading e-book subscription service and the fifth-largest e-book retailer in the U.S. Half of its titles are self-published. Subscription price: $9.95, free 30-day membership. Royalties: Authors receive about 60 percent royalty via Smashwords. BookBaby authors in their Premium package earn 70 percent from Oyster with BookBaby earning 0 percent. Authors in BookBaby’s Free or Standard package earn 70 percent minus 15 percent to BookBaby for a total of 55 percent of the book. Like Scribd, self-published authors need to go through Smashwords or BookBaby.