Writing a booking that explains the hidden power of drupal apis. Example each api with a quick working example. The most needed explanation is how to deal with users, roles and permissions. how to deal with ubercart apis. Only 3 books exists that is aimed at programmers and to name them. Pro drupal 7 development by Tomlinson and VanDyk, Drupal 7 module development by Matt Butcher and others, Drupal 7 themes by Ric Shreves. This is in relation to drupal 7. These three books are not enough even though they are great.
Can some of the guys in drupal development team write more books for programmers.
Also organic group is very interesting and more on its apis is needed.