For a long time I was thinking about write or not a review of the Drupal 8 Module Development (the former edition of the current). For me there were two very important keys: on the one hand, it was a very ambitious book in terms of scope and content (so it was an important challenge to make a synthesis on its simple review). On the other hand, Drupal 9 was already on its way and it was possible that it would be deprecated quickly.
These days, my life is all migrations, all the time, which means I often need to run Drupal 7 and Drupal 9 sites side-by-side simultaneously to compare the results.
The latest version of Drush, Drush 10, only works on Drupal versions 8.4+. To use Drush on Drupal 7 sites, you need an older version, Drush 8. And both of them use the command drush. Tricksy…