Menu link to block or div
This is not completely straight forward. If you try to create and save a menu link like book/page#anchor-id, Drupal will give this error


  The path 'book/page' is either invalid or you do not have access
  to it.


You will need the node ID. Drupal wants the link to look like this: node/55#block-you-want.

The easiest way to do this is


Save the menu link without the anchor id. ex: /book/page
Drupal will create the menu link like this: node/55
Edit your menu link to: node/55#anchor-id. Save again.