diff -u b/core/modules/book/templates/book-navigation.html.twig b/core/modules/book/templates/book-navigation.html.twig
--- b/core/modules/book/templates/book-navigation.html.twig
+++ b/core/modules/book/templates/book-navigation.html.twig
@@ -32,7 +32,7 @@
  */
 #}
 {% if tree or has_links %}
-  <nav id="book-navigation-{{ book_id }}" class="book-navigation" aria-label="{{ 'Book Navigation'|t }}">
+  <nav id="book-navigation-{{ book_id }}" class="book-navigation" aria-label="{{ 'Book navigation'|t }}">
     {{ tree }}
     {% if has_links %}
       <h2 class="visually-hidden">{{ 'Book Navigation'|t }}</h2>