I think they base the 90% on what "publishers and distributors
" told them.
points to 60% Kindle share of the e-reader market, but at least some people buy ebooks from Amazon even if they own a different reader, because of the lower prices. Interesting factoid about Christmas day ebook sales outnumbering pbook sales:
"... On Christmas Day, for the first time ever, customers purchased more Kindle books than physical books," Amazon said.
Amazon e-books can also be read on Apple iPhone or iPod Touch devices. The Kindle online store boasts a library of 390,000 digitized books for sale.
While Amazon has not released figures for Kindle sales, Forrester Research estimated in October that the Kindle has a nearly 60 percent share of the US market, followed by the Sony Reader with 35 percent....