It’s not possible right now. Each library that’s syncronized needs to be built internally so it can link with Kodi’s internal database. For now they’re only accessible by going through the add-on menu. We have an open issue to track adding them, but haven’t gotten around to implementing it yet https://github.com/jellyfin/jellyfin-kodi/issues/222
Books are a bit of a strange case. We’re limited by Kodi’s internal media types, which is currently Movies, TV Shows, Music, and Pictures. If we do enable the syncing of books, I believe only audiobooks would be possible (not standard ebooks), and it would get merged with a music library, which may not be what is wanted. I’m leaning towards not implementing audiobook sync for that reason, but by no means has a final decision on that been made yet.