If you decide to make your book exclusive to Amazon and enrol in the KDP Select program, you will need to ensure that the eBook does not appear for sale on any other website at all. Amazon will take your title off sale completely if the eBook is showing on any other website during a KDP Select period of exclusivity.
You can list on Amazon without enrolling in KDP Select, and also make the eBook available on other retailers, Amazon does not demand exclusivity unless you enrol in KDP Select.
KDP Select does not work for everyone, it pays to research thoroughly before making your title/s exclusive. Read the terms and conditions very carefully, and see what other authors are saying about their success or otherwise on KDP Select. Read (recent) blog posts from authors in your genre, or from independent authors and organisations monitoring trends in self publishing.
Update to post: Author Catherine Ryan Howard includes this helpful post on her website, which is a fabulous resource for writers.
ARE AMAZON *REALLY* PAYING AUTHORS PER PAGE READ? NO. NO, THEY’RE NOT. [PAUSE] WELL…
Keep in mind that an established author with many titles, can have a completely different result in KDP Select, to a first time author with one title. Our publisher clients may test one title in KDP Select, but keep other titles on general release. A traditionally published author might publish a backlist title independently and make this Amazon exclusive, taking advantage of a free promotion on the backlist title and use this as a loss leader to promote other titles.
Simply listing in KDP Select is no guarantee that Amazon will give your titles preference in search results, or book suggestion spots. Amazon (and other retailers) will promote titles that are selling well, rather than lifting new or slow moving titles from the shadows and helping them along. The only way to sell eBooks successfully is for the author/publisher to take responsibility for promotion, taking every opportunity to spread the word through all available channels.
Many of our clients consider KDP Select after the eBook has been on sale for a while, to see if the promotional opportunities make a difference to sales. In order to meet the conditions of KDP Select, the book must then be delisted from all other retailers.
Most of the retailers will respond quickly to a request to take an eBook off sale, but some do not. We have experienced a delay of 6-8 weeks when attempting to delist an eBook in some instances, because one of the smaller websites served by Gardners for example or Ingram, has not deleted the listing.
If you think you may wish to try KDP Select at some stage, we suggest that you do so before extending your distribution reach beyond Amazon. Alternatively, restrict your availability to Amazon, Apple, Kobo, Google, Overdrive, B&N who will all respond quickly to a request to remove an eBook from sale. This will give you more flexibility and access to a reader audience beyond Amazon, with the option to try KDP Select if you wish to at some stage.