Yesterday MacRumors reported that “a new message has been spotted in the Photos app on macOS 10.13.6 [which] states that final purchases for the built-in photo printing service will need to be placed by September 30, 2018. Afterwards, it appears that Mac users will have to opt for third-party companies like Shutterfly to craft physical books made from their digital photos.”

This is very sad. A few years ago, with Aperture, a top-class app, one could compose a photo book with great flexibility and precision, and readily get it printed by Apple at a high quality. i have made a bunch of 100-page large-format hardback books.
The elimination of Aperture was the first sign that Apple was dropping any interest in serious photographers. The elimination of these books is the last step.
I had actually been using workarounds for several years in order to continue to exploit my hard-learned Aperture skills: I, and many other in these forums, successfully made Aperture work in new OS’s up to High Sierra. In a more complex procedure I and others here were able to convert a book composed in Aperture to pdf’s and then use Photos to print it. The book design options in Photos are severely dumbed-down and not a patch on Aperture’s design flexibility. The printed books from Photos with transferred pdf’s were almost as good as the Aperture ones (I did find a slight decline in quality, with less deep blacks, and color variation when I re-ordered a book).
From the very beginning, the Mac has attracted creative and visually-oriented people. The deliberate cancellation of Aperture and Photo Books shows how far the present management at Apple have drifted from their core supporters and enthusiasts. Simplified (dumbed-down) apps for the masses are the way they want to go, it seems. Forget creativity and quality.