Something more in-depth than the handbook page at http://drupal.org/node/72108 with outlines of approaches for different Drupal competencies.
philosophy of troubleshooting website problems in general
assumption of lots of moving parts
you're not building something, but rather something went wrong, and you're trying to figure out why
which layer to look at first?
how to recommend a fix?
where to look for logs, and what the logs mean
documenting for the next time something like it comes up
throw it on Drupal.org no matter how esoteric
you don't need to use client names, website URLs. example.com, etc. is usually fine
what you did
what you expected
what happened instead
what you did to fix it
a pamphlet per Drupal competency
modules (PHP)
Views
themes
Panels/Contexts/etc.