Q: Can't buy book on my iMac

What system and iPhoto versions are you running? What are you experiencing when you try to order the book?

Can you create a PDF preview file of the book as described in this Apple document: iPhoto, Aperture: Previewing an order in iPhoto or Aperture?

If not boot into Safe Mode, Mac OS X: Starting up in Safe Mode, and try to create the book there.  If you can, check it out carefully for any errors and then order the book from the Safe Mode.

Save the pdf file to compare it with the printed copy when it arrives.

OT

What is your iPhoto version? You need iPhoto '09 or '11 to order a book (see: Apple Photo Services: Product line details for books, cards, and calendars)

Try to order your book in Safe Mode (see:  Mac OS X: What is Safe Boot, Safe Mode?http://support.apple.com/kb/HT1564)