What about 16/10 ?
It's the closest from the golden number (1.618...).
By the way, I think most e-paper reader are 16/10, aren't they ? (Sony, Kindle, PocketBook, etc., according to their resolution.)
I dislike 16/9 for other screens than a TV.
The best for A4 would be... 1.414 (by order of preference: 14/10, 10/7, 13/9, 4/3, 16/10, 16/9).
For US letter, it's 1.294 so ~ 4/3.
However, there are plenty of p-books that are not "standard" size...
For readability , a text shouldn't be more than about 80 characters per line.
An interesting link:
http://en.wikipedia.org/wiki/Canons_...e_construction
Finally, an economic consideration: for manufacturer coupled to marketing, it's far more interesting to have a big aspect ratio. By claiming the same screen diagonal, one can strongly decrease costs in raw materials.
To put this bluntly, one could sell a screen with a diagonal of 100" with only one pixel width...
We consumers should ask for indication in term of surface + aspect ratio (or + diagonal), rather than diagonal + aspect ratio.
Thierry