It starts off very open, but you have to have a good memory or at least be willing to flip back for a few of the earlier references by the end of it. Anything too difficult is laid out in a picture then explained, which is a good format for it
Simon Singh was a colorful character as a speaker on the cryptography panel at the NYC World Science Festival, though very detail oriented (I hear that a good personality trait for this field). I've been meaning to check out his books - thanks for the rec.