When is a Book Being Read "Finished"

When do you consider a book you're reading as being finished? So many books have extras at the end - study guides, interviews with the author, endnotes/references, etc. I usually read the author bio and acknowledgements, and if there are endnotes/references with anything more than a source attribution or citation. I won't do much more than skim other things. So have I really finished reading a book if I haven't read all of this extra stuff?

Thoughts?

WhiteStix