P A R T V
Chapter 21 covers porting a Drupal 6 module to Drupal 7, which can be a great way to learn module development.
Chapter 22 provides another good onramp to writing modules—“glue code,” or site-specific modules, to make the final tweaks you cannot quite achieve with configuration. This chapter can be read without reading any of the previous chapters first.
Chapter 23 covers writing tests for your module, an essential part of reliable and sustainable code.