To me, "incredible" books are those that I can recall years after having last read them. For example, I consider the historical fiction by Shayne Parkinson that begins with
Sentence of Marriage (free and available at Smashwords among other places) to be incredible because more than 2 years after having read it, not only can I recall the story in detail but my wife, myself, and several friends still talk about the book. Same is true of Harper Lee's
To Kill a Mockingbird, which I last read in the 1960s.
My initial recommendation for the incredible ebook read remains Shayne Parkinson's quintet of historical fiction. To those I would add the mysteries of
Vicki Tyley. Also excellent and approaching incredible but not quite there are the science fiction/fantasy stories of
Michael Hicks and
Tracy Falbe.
Another outstanding series of books are David Weber's
Honor Harrington series. These waver between incredible and nearly so. As with most series, some volumes are better than others, but many are in the incredible category. Many of the books are available for free from the Fifth Emperium on the
Mission of Honor CD.
(BTW, I do not interpret incredible to mean of great literary value or likely to be literary classics 50 years from now. I equate incredible with incredible read.)