Maybe this is part of the reason for the decline :
https://www.amazon.com/Triple-Crown-...=felix+francis
A hardcover for less then 3$ more than a digital book ?
But let's look some more -
https://www.amazon.com/Triple-Crown-...=felix+francis
Hard cover - 37.99$. Paperback - 11$. Kindle - 28.49 $ (!).
(prices of course may/will change for each viewer, but that's what I see now. Also note that this are 2 sales for the same book with totaly different prices).
Why buy a digital book ? (And I'm not even talkng of the economics of printed, physical books vs electronic ones).
Add to this the problems of DRM, inability to share, etc. and no wonder.
On the other hand they are not counting me (Delphi Classics) or my daughter (Gutenberg) and I guess many more like us that don't necessarily need to read the latest bestseller and will settle with a classic or an indy.