According to Apple, most print projects will migrate to Photos and can be re-edited and re-ordered there.
The exception to this are wire bound books and small softcover books, as these are no longer supported in Photos at this time.
If you've made a wire bound book, the images originally used for that book will be preserved as an album in Photos, so you'd have to re-create a new book with those images.
More information on print products in Photos is available here.
Do you still have iPhoto installed and if so what version?