If you are using Pages v5(.2), you can select the Document icon from the Toolbar, and temporarily set the Paper Size to A5. Your document content will reflect this change. You can then export to PDF, ePub, or using the File > Print > PDF sequence, you can add the PDF to iBook. If you get the free Create Booklet PDF Plug-in, you can also do File > Print > PDF > Create Booklet. It still works on OS X 10.9.4.
Another tool that does imposition on an existing PDF and produces a booklet is the shareware ($35) Cheap Imposter. I just downloaded and tried it on OS X 10.9.4 and it seems to work. It provides the ability to change the Paper size, margin offsets, and document zoom factor. The preferences for Cheap Imposter optionally allow you to install a PDF Print service, so that it works like Create Booklet above.