How to create structure "Author -> Book -> Quote"?
Complex business like this is hard "out of the box" with Drupal. Everyones custom logic like you've listed is different -- so community modules regularly dont let you define just deep nesting of logic.

I suspect you'll have to create some custom code to enforce relationships you want (form  validation logic).

Additionally, look at Conditional Fields, Entity Inline Form and perhaps the Prepopulate module.

You can use these modules to build the dependant drop down select lists, and a button to create the Book or Author for a quote as needed -- the prepopulate module could let you pass info from a book or author on creation, to the reference entity.

You may want to look at Display Suite or Renderable Elements to tweak how the node form(s) are layed out.

As I said this isn't a solution but tools to help you build your own business-logic solution.