Instead of using Calibre's objects I find it simplest to use the Python library ebooklib. Calibre's container types work with exact MIME types whereas ebooklib simply lets me ask for all ITEM_DOCUMENTs in an ebook. Here is some sample code I have written which demonstrates using it to read ebook contents:
Code:
import ebooklib
import lxml
book = ebooklib.epub.read_epub("path/to/book.epub")
docs = list(book.get_items_of_type(ebooklib.ITEM_DOCUMENT)
# beware non-UTF8 content! E.g. you might need to .decode("latin1"), or some other encoding, instead.
doctree = lxml.etree.fromstring(docs[0].get_body_content().decode())
If you are interested in counting words then I recommend Calibre's calibre.spell.break_iterator.count_words function which reuses logic from the International Consortium for Unicode to get it “right” (± locale and quality of input text).
Good luck with your project.
