Well, without more context, it's hard to say precisely. Maybe it's a novel that uses eucalyptus as a symbol. For example, it could represent something like renewal or mystery. It might also be a story centered around the people who work with eucalyptus, like loggers or botanists, and their experiences and relationships.
I'm not sure specifically as I haven't read a novel named 'eucalyptus novel'. It could be about eucalyptus trees, perhaps their ecological significance, or it could be a story where eucalyptus trees play an important part in the setting, like a mystery set in a eucalyptus forest.