There is no question that Drupal is growing, largely through word of mouth. Yet, go to the computer section at a well-stocked book store, and still there is precious little on the shelves about Drupal.

But breakthroughs, both large and small, are like that.

When calculus was first introduced, there were no text books on what Newton and Leibniz had discovered. The knowledge was largely word of mouth and it took one of the Bernoullis to put together a first pass book about calculus. It would be hundreds of more years before Thomas' Calculus would flood every modern scientific school of higher learning.

Part of what we are seeing is a moving target.

I once heard a graduate student put it this way,

The text books are at least a couple years behind the latest advances.