The links below are to articles reporting on the ebook subscription space, with Kindle Unlimited coming out on top currently.

For more visit:
– http://the-digital-reader.com/2015/05/04/amazon-is-killing-it-in-the-subscription-ebook-market/
– http://goodereader.com/blog/e-book-news/kindle-unlimited-is-the-most-popular-e-book-subscription-service