How do I reorder a Photobook?

The re-order option used to be there in iPhoto and Aperture for a limited time after you placed the order, but I have not seen it in Photos.

Just upload the book again and place theorizer again. You would not save any shipping costs by ordering a second copy anyway, only the long upload to the store.

Apple used to keep the PDFs around for a few weeks, until the book has been delivered, This way they could check the original PDF, if a customer complained about the print quality. Perhaps it is just too late to order the book again. You could try to use the "report a problem" button on the Order Status page for your order and ask the store directly, if you can receive a second copy.

To help you with future books I suggest you create a new library for each book. You can assemble the photos for the book any way you want, in a folder on the hard drive or an album in your Photos library and then add them to the book library. This will give you a relatively small library that will be easy to store elsewhere so additional books can be ordered at a later date if so desired.