Do you actually read both sides? On a textbook, magazine or encylopedia, perhaps - mainly because a physical book doesn't have search features.

On a novel, it's portrait - you need paragraphs of context as you scan forwards and backwards to follow the information stream.

On a computer with text search facilities, it's more convenient to have vertical text results and paragraphs - as long as the width is acceptable.

Of course, TVs look better in a widescreen format, and computer screens have been forced to orbit TV display sizing for the past decade... and this is why we're stuck with useless 16:9 screens with not enough vertical pixels.