Manufacturers started to produce those wide computer monitors to SCAM consumers. They claim it is because consumers demand this, but the reality is, that 17" 4:3 monitor has larger screen area than 17" 16:10 monitor. (*)
16:10 monitor might be better for viewing wide-screen movies. But for most computer applications with system menu at the bottom, status line, windows title, application menu at the top, toolbars, or even ... shudder ... ribbon 4:3 will give you more text in a word processor, more numbers in spreadsheet, more books in Calibre, more code in IDE, ...
For books, 4:3 just looks better to me ;-)
This might be because I have been using 4:3 monitors for the last 20 years, and 4:3 TV even longer.
A4 has ratio 1.412:1, which is closer to 4:3 (1.3333) than 16:10 (1.6).
Books here generally do not use A4 or A5, but B or C series format. All those formats have so-called Silver ratio 1 to (square root of 2) = 1:1.412
See
http://en.wikipedia.org/wiki/A4_paper#The_A_series
(*) See a little math here:
4:3 monitor with 20 inch diagonal has:
Dimensions: 16x12 inch
Area: 192 square inches
16:10 monitor with 20 inch diagonal has:
Dimensions: 16.96x10.6 inch
Area: 179.775 square inches