app/views/books/_book_nav_links.html.haml
.col-md-12
%ul.nav.nav-tabs
%li{ class: is_active(activated_page, 'show') }
= link_to 'Book', language_book_path(book.language, book)
%li{ class: is_active(activated_page, 'edit') }
= link_to 'Edit', edit_language_book_path(book.language, book)
%li{ class: is_active(activated_page, 'history') }
= link_to 'History', history_language_book_path(book.language, book)