Drupal 8 Remove book child pages with in the content block I am looking to not show Book child-pages within the content and only show them in sidebar region. So I anticipate not to hide them using css but remove the child-pages that gets called automatically within the main content (near to the end of the content block)
In D7 (not sure of D8) if you install Display Suite and enable the "Full Content" view mode for Books. You can then apply any layout you want to book pages and control where the book navigation goes for example.

Attempting to override the book behavior from Core can be very difficult ...