How to download Photobook product extensions?

Apple has closed down all printing services for books, cards, calendars. Last orders had to be placed by September 30, 2018.

If you created a book in Photos, using an older system version, where you can still create books, try, if you can save the book as a PDF file and send to PDF file to a printing service, that can print Apple Books from PDF files to save your work. In 2018 these print services have been available for PDFs of Apple Books. I am not sure, if they are still available:

• • Mimeo used to print for Apple and can handle PDFs created in Photos, see: https://import.mimeophotos.com
  • Also Presto Photo can print from Apple https://www.prestophoto.com
  • If you are not located in the US, there may be local printing services, which would be more economic with respect to international shipping fees and shipping time. In Germany we can use photographerbook to print existing Apple PDF files: https://www.w2p-photographerbook.de/fotobuch-aus-aperture-iphoto-fotos

To be able to install project extensions, where you can create books and have them printed, you need at least macOS 10.12 Sierra. Ordering Books and Calendars on a Mac that cannot be upgraded past El Capitan… - Apple Community

How did you create the photo book?

Normally, you would need to download the extensions first, then use them to create the photo book.

In any event, you can download extensions for photo books through the File menu:

File > Create > Book > App Store....

That will take you to a special section of the store for extensions for creating and editing photo books, cards, calendars, wall art, and other printed materials.

If you have already created a book, you would have to find the extension that matches whatever you used to create it.