Indeed. And the other drawback of hierarchical organisation is that you can only present the structure from one viewpoint. In other words, if your subject matter has regional content (for example) plus seasonal content do you organise your hierarchy first into Americas, Asia, EMEA and then each chapter into Spring, Summer, Autumn, Winter. It would be equally valid to do it the other way round (season first then region). Neither is useful unless the reader knows what to expect.
The depth of the hierarchy generally reflects the number of dimensions in play. The authors will often find the same sub-headings repeated in many different chapters.
With online material we have other options: to present all dimensions as options with equal weight (a Season radio button and a Region radio button), or for a more general solution we could simply enable search and have a Google-style interface to the material.
If the search is backed up by a genuinely useful index that reflects the many variations allowed by the content's language (do I search for Fall or Autumn for example) then this solution is often usable.
What can help enormously is to allow the content to be tagged by its consumers. Then the index becomes more useful over time as people find the text they are looking for and flag it using a term that is meaningful to them. This technique contains a positive feedback mechanism that eventually takes account of common spelling mistakes and other "errors" simply by leveraging the fact that previous readers have made the same mistakes.
People are experimenting with "tag clouds" and other devices for portraying this information. It will be intersting to see what standards emerge over time.
-- Dominic Sayers (email)