Derren Brown: tricks of the mind (and page)

I'm currently reading the surprisingly well written and interesting book Tricks of the Mind by British magician and mentalist Derren Brown. At one point in the book, Brown confesses he is:

...never one to arrive at an acumen regarding a set of printed pages bound along one side, based purely upon my discernment of its sheathing, ho ho

Ignoring for the moment the interesting use of the word 'acumen', my question to you is: what on earth is he talking about? I'll post the answer tomorrow, although I'll doubt you'll need it.