One of the most powerful features of Drupal is "book" module. If you have organized your site content into drupal books, then make sure you enable the book navigation block.
Goto Administration >> blocks >>
Enable "Book Navigation" block & place it on the left side with 0 ordering.
Whenever a user browse the book or book pages, the left side column of site will show the book navigation block. Just like on this site while you are viewing this page.