A family of Microsoft word processing software products for creating web, email, and print documents.
Are you using the "Book fold" setting in Word? If so, the orientation automatically changes to Landscape, and the document will print two portrait pages on a landscape page—normally A4 or 8.5" x 11". If your paper size is larger or smaller, you will need to adjust accordingly (and of course your printer must be able to handle it. And the number of pages in the book must be a multiple of 4. (More at http://wordfaqs.ssbarnhill.com/2PagesPerSheet.htm#CreatingBooklet.)
Note that Word is notoriously bad about printing selected pages from a "Book fold" document. Sometimes a better approach for a document that is to be printed is to set the paper size to the actual page size (A5, for example, or 5.5" x 8.5") and then save the document as a PDF. In the PDF Print dialog, select "Booklet."