It's probably HTML with CSS...
I think this will be a mess for quite some time... Most e-readers are limited technology. Limited by screen size, resolution, colors, battery, reflectivity and a whole lot of other minor technological issues.
Can you imagine the web as it would have been in the days of the ALTAIR or maybe even later in a world with C64's and Sinclair Spectrums?
Sure, yes, it would have worked, but how can the web of today even compare to the BBS's that spawned it. The inexorable progress that despite having the weight of the world behind it still took two decades...
In the e-reader world, we're at about the C64 level of product evolution.
What will this bring in a decade? Maybe an e-reader will be entirely virtual. Seen through glasses and imposed over our reality - an amalgamation of augmented reality and e-paper.
Will it be something we read directly into our mind? The first stage of implants intended to bring auditory and visual hallucination. Then we might well read a paper book that doesn't exist, turning the page with fingers that aren't real.
Even more realistic changes in format to increase size to around A4 size and reduce weight while increasing battery life will be a huge improvement. Dedicated E-readers will merge with powerful low-power processors and define a new generation of computing.
The old 5 to 8" book format e-reader will probably conform to a standard "E-File compatible" or something, being given to you as a Christmas present by an Aunt you haven't seen in three years, complete with three and a half million books, all free content.
It will probably even get to the point the older, proprietary readers won't even have enough value to pawn off. You'll find them laying on the side of the road, sitting on a bus-stop seat where the batteries failed just one too many times.
Give it time and things will change... Then look out for Sony or Microsoft or someone else to start touting the new generation of e-readers...
"We've got them already."
"Yes, but this one plays video at four times the resolution!"
Nothing really ever changes does it?
David.