incidentally, for booklets under 16 pages BookletCreator is a nice little app for Mac & Windows. The paid version can handle more pages, the trial is limited to 16.
There's another windows app I've used which is more versatile, but has a slightly higher learning curve. Can't remember what it's called, or whether other platforms are supported.
(obviously lots of apps let you manually reorder pages -- it's the automation that's trickier to find)