Dietz S...
Peggy is right on the money...but I think she was misunderstood.
Cocoa Booklet is a plugin for OS X for AFTER you're done designing. It's sole purpose is to take the page layout that you've been working on and conform it to what is called "printer's pages" for use in a saddle or stitch bound "booklet."
Take a playbill apart and you'll see that there are actually FOUR pages of content printed on a single sheet of paper...and if your pages are numbered, you'll see that the outmost sheet of a 10 page booklet will have the front cover and back cover on the outside, and pages 1 and 10 on the inside. The next sheet will have the contents of pages 2, 3, 8 & 9...and so forth.
Cocoa Booklet does this for you after you export your layout from Pages, Word, or what have you.
Hope that helps!