You'll need to use Create Booklet Service to create a booklet with any number of pages (best in a multiple of 4). You can create the booklet directly from the print dialog after you install the service. Again, for US letter size booklets, I find it best to create them in legal size as above. Download it from the link & run its installer. It will open the booklet in Preview. If you're using Lion, I suggest using File > Export to save the PDF where you can find it.
For a folded US letter-size booklet start with your Pages document in Legal size & larger than normal font size. Export the Pages document to a PDF & then drag the PDF to the icon of Cocoa Booklet. It will create a new PDF in booklet form. For international paper sizes, create the Pages document in the larger size with a larger font, i.e. create A4 pages to create A5 folded booklets. You can also use US tabloid or A3 to create larger booklets.
