If you bought Aperture recently, I assume you downloaded it from the AppStore? Then it should install automatically - there is nothing else to do. Or does Aperture not install at all?
I install Aperture to assist me with my IPhoto Project on a Wedding Book. HELP!
Or is your problem with a photo book you started in iPhoto and now want to continue in Aperture? Unfortunately, photo books are one of the library items that are not yet shared properly between Aperture or iPhoto, see Aperture 3.3: Using a unified photo library with iPhoto and Aperture
Books, calendars and cards you create in iPhoto are visible in the Source list in Aperture, but cannot be edited. Books, web pages, and web journals you create in Aperture are not visible in iPhoto.
So, if you start a book in iPhoto, you will have to finish it iPhoto and vice versa.
You could copy your photos to a simply album in iPhoto and then create the book new in Aperture, if you rather want to create it Aperture.