@ned,
The WXR Import routine gives the user the option to select “Front Matter”, “Page”, “Part”, “Back Matter” and it imports those post types correctly. The EPUB import routine only allows the user to select “Front Matter”, “Page”, and “Back Matter”. It would be an awesome feature to allow users to select “Part” and even other post types (a filter hook already exists called pb_import_custom_post_types
that is used in the WXR routine).
Having not spent a great deal of time looking at it, it seems plausible to be able to do this.
Cheers!
Bryan