When a book first comes out I usually enroll it in KDP Select for the first few months. That makes it available to people who use Kindle Unlimited, but it means it’s not available for purchase anywhere outside of Amazon. I did this with E is for Evil, which means until recently it’s only been available for readers who use Amazon but–good news everybody!–it’s recently moved out of it’s KDP Select term and is now much more widely available.
So, if you weren’t picking up a copy of E is for Evil because your favourite distributor didn’t have it, now might be a good time to change that!