A quick googlegoogling of 'writing drupal modules' returned:
Or the following developer guides:
http://drupal.org/node/231276 for Drupal 6 modules, and,
http://drupal.org/node/361112 for Drupal 7 modules.
There is also a moduleproject, http://drupal.org/project/examplesExamples for Developers, thatwhich gives lots of insight into module writing and best practices, etc. ItIt has versions for multiple releases of drupalDrupal as well.
With more respect to .inc files, the documentation for http://api.drupal.org/api/drupal/includes%21module.inc/function/module_load_include/6module_load_include() explains how Drupal includes them within modules, but basically. Basically they are drupal'sthe Drupal's way of breaking up modules into more manageable pieces.
Otherwise, a complex module might have a gigantic .module file.
Also, sometimes modules don't need all of their code loaded,loaded; so, code that is only called in certain routines is delegated to a .inc file for performance reasons and loaded only when needed.
If screencasts are more helpful, you might want to check out http://2011.drupalcampla.com/sessions/your-first-module-mostly-painless-intro-drupalYour First Module -development A Mostly Painless Intro to Drupal Development and click on the screencast link. I'd include it, but it has an ugly url that doesn't encode here well.
Bob over at http://mustardseedmedia.com/podcast has some great podcasts as well, butwell; most of his are D6, but he is getting more and more into D7.