Why are books taller than they are wide?

The answer is because animals are longer than they are wide:

“[M]edieval manuscripts, like modern printed books, are usually taller than they are wide. This is inevitable if one begins by folding an animal skin which is naturally oblong. When paper was introduced for making pages of books, the makers tended to follow the same custom, though it was no longer essential. Books are still taller than wide: this is because, more than six hundred years ago, their ancestors were made by folding natural vellum.”   (Christopher de Hamel, A History of Illuminated Manuscripts, 89)