Synchronising uploaded files across devices in Books

I can’t get audio books uploaded from cd to Collections in Books on my Mac to show on other devices in Apple Books. I have followed the advice for settings on my Mac and my IPhone, but audio CD books that I have uploaded to Books on my Mac are not visible on my iPhone. I suspect that Apple doesn’t want me to do this because they don’t make any money from it, but hopefully it is just because of my techno-incompetence. Any advice would be appreciated