Amazon Kindle Select is a relatively new program launched to help independent authors and to help Amazon's sales. It also is a way for Amazon to dominate the e-book industry by requiring independent authors to digitally publish their books exclusively on Amazon Kindle Select. Authors cannot have the book published on any other platform while enrolled in Amazon.com's Kindle Select program. The other hitch is that your book will be available for free during the promotional period of time, and lastly, your book will be available for checked out in the Amazon Kindle Owners' Lending Library. The Kindle Owners' Lending Library can only be accessed by Amazon Prime members.
The best benefit for independent authors in this program is the 5-day free promotion period every 90 days. During your free promotion days, Amazon will promote your book on their site and offer it for free for the days you select. You can either use all 5 days consecutively or you can schedule them sporadically throughout the 90-day period. Cheryl Kaye Tardif who wrote How I Made Over $42,000 in 1 Month Selling My Kindle eBooks broke up her five days into two promotion times: a three day promotion and then a two day promotion later on. It worked for her. Her book tells how she laid out a marketing and promotion plan for her book using Kindle Select. The book explains how her sales increased over a short time as a result of the program. It was impressive.
So if you're interested in enrolling your book to Kindle Select, you'll first have to remove it from other platforms like Smashwords, Apple QuickReads, and Barnes & Noble, etc. But it could be the best move you make in getting your e-book ranked higher in the lists and increasing your sales.