Add book and part pages to Text output
If there are explicit parts and/or books, they should be included in the output. Remember, if the title is None, it means it is implicit. If there is an empty title, it means it was explicit, but without a title. Implicit parts and books should not show up in the output, but should explicit ones without titles should just be numbered.
For Text, the start of each part and book will just need to be noticeably offset from the preceding text, and probably the subsequent text a bit as well.