Installed (from the same download) LR 6 on Mac Pro 2010 - OS X 10.10.5 and on Mac Book Air 2011 - OS X 10.10.3. On Mac Pro 'book settings' in Book-module is missing, on Mac Book Air 'book settings' is present and working. Where to look for solution? Help is appreciated.
Right-click (control-click) on one of the other tabs such as Auto Layout, Page or Guides and re-select Book Settings from the list.
In the book module you can also choose it from the menu bar:
Window >> Panels
Once you know it's easy!