A4 and US Letter
Between Borders has an interesting 2002 article on paper sizes. The differences between US Letter and A4 may not immediately appeal to everyone as exciting reading, but Brian Forte’s article on aspect ratios (A4 is always 1:√2, while Letter alternates between two ratios), starting sizes (A0 is almost exactly 1m² in size, while letter is arbitrary) and photocopier settings (the 71% reduction option is for reducing A-sizes) is actually quite fascinating.
The full article is here.
Further reading
These posts may be related to the one above:

