app/views/layouts/application.html.erb

<!DOCTYPE html>
<html>
<head>
  <title>ShareYourBooks</title>
  <%= stylesheet_link_tag    "application", :media => "all" %>
  <%= javascript_include_tag "application" %>
  <%= csrf_meta_tags %>
</head>
<body class='<%= controller.controller_name %>'>

<%= yield %>

<% if session[:user_id] %>
  <%= button_to 'Logout', logout_path, method: :delete %>
<% end %>

</body>
</html>