Hi everyone!
Same problem here since few days, in Photos 1.5 and OS X El Capitan (10.11.6) on a mid-2015 Retina 15" MBP. All was working fine one or two weeks ago, but since last week-end (at least), Photos crashes when I click on "Buy the book" button. I've thought about a problem with corrupted photos or Photos library but the exported PDF of the book is OK, without any bug, and I've tested with some former already printed books (ordered last week and one month ago) and Photos crashes too with these orders (I did not encounter any problem during ordering these books at the time).
Problem disappears when I select a different country in Photos' preferences, but I can't order because my billing address is located in France.
Many feedbacks and crash reports have been sent to Apple, but no news and the problem persists...
Hope Apple will resolve this quickly!!