By "The Book", I assume you mean "Rocket Mass Heaters: Superefficient Woodstoves YOU Can Build" by Ianto Evans. On page 16, yes it shows the "push and pull" effect. And on page 24, he says you can have an eccentric gap for more efficient direction of radiant heat. The diameter of the inside of the riser is dependent on the diameter of the burn tunnel and exhaust pipe, which should have the same cross section area through-out.