As I expected, my estimate that 30% of the population might benefit from a dedicated reading device looks a bit high.
This vintage IPSOS report from the time of the Kindle introduction (2007) is a nice snapshot of people's reading habits:
http://surveys.ap.org/data/Ipsos/nat...%20Topline.pdf
In a Kindle-less world, 27% of the population read zero books in a year and another 37% read less than 7. For a total of 64% reading less than 7 books. Only 27% of the sample reported reading at least one book a month.
(Note that the listed percentages are fractions of the 73% that *did* read, not of the sample.)
I expect that the mainstreaming of ebooks has since changed people's reading habits but doubt that it has turned many nonreaders into readers or casual readers into avid readers. Throw whatever fudge fact you like but I doubt even somebody that reads one book every two months is very likely to drop even $49 on a dedicated reading device.
So the most recent ereader penetration numbers (22% was it?) has already captured the equivalent of 80% of the book-a-month population. Definitely not much room for explosive growth, there.