I am using both the sidebar-first and the sidebar-last.
The book page navigation at the bottom of the content area is then split into 2 lines.
The

and the are in the correct location. The is situated correctly at the rigth side but one line lower. When the width of the screen is decreased and the content area gets smaller, also the goes to the second line. This does not happen when there is enough content present in the book-page so the contents ends below the bottom of the sidebar-last.