Hey janthwaites,
I understand that you are having some issues with your ability to order a photo book currently. I have an article with some suggestions on resolving this issue, and it can be found here:
iPhoto: Difficulty submitting a book, card, or calendar order - Apple Support
https://support.apple.com/en-us/HT201170
You may also want to try previewing your photo book to make sure it looks correct and there are no conflicts before attempting to order. Use the information in the following link to accomplish this:
Preview a print project in Photos, iPhoto, or Aperture to avoid issues - Apple Support
https://support.apple.com/en-us/HT201504
If you are still experiencing issues, try force quitting iPhoto, then boot your computer into Safe Mode using the steps in the following article, then attempt to order the photo book:
Try safe mode if your Mac doesn't finish starting up - Apple Support
https://support.apple.com/en-us/HT201504
Take care, and thanks for visiting the Apple Support Communities.