I also opened a bug on this on the Apple developer bug-tracker. It was closed with this comment:
Engineering has provided the following information regarding this issue:
Syncing support has been removed for books from iTunes 12.7 but you can manually add your books by dragging and dropping (even multiple books at once) from a folder on your computer to your device in iTunes 12.7 for Windows. This is detailed in the following support article (scroll to the bottom of the page to the section titled Manually add items from your computer)
https://support.apple.com/en-us/HT201593