book-rtl.css 

.book-navigation .page-previous {
  float: right;
}
.book-navigation .page-up {
  float: right;
}

Archivo

drupal-6.x/modules/book/book-rtl.css
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  1. 

  2. .book-navigation .page-previous {

  3.   float: right;

  4. }

  5. .book-navigation .page-up {

  6.   float: right;

  7. }