p866:

  The level of indentation that seems to produce optimal 
  results in comprehension is between 2 and 4 spaces; as the 
  number of spaces increase, the comprehension level 
  decreases.

On the other hand, the sample program hard parts that were indented nine times (!) in places. It's an interesting read, but I'd not try to use it in a discussion.

Personally, I don't care; just pick one (hopefully one that matches what most people are using with that language, eg. 2 spaces for Ruby), and get on with it.