I would think that since the contents was a book that it would qualify for media mail and that the outside doesn't matter too much as long as it is mailable. 

The PO does not care what you put the book IN period.   Unless it's a priority box, then they charge you extra.   You can mail things in a tube, in a sealed bucket, and yes in a purse as you found out.  lol    Actually you can just wrap the book in a plastic bag and tape it securly and mail it.   I wouldn't but you COULD.  

Yesterday I mailed stuff in an old Crystal Light container.  It was mailed Priority.  The clerk didn't even blink an eye. 

I've mailed books in all kinds of things:  outside bag of Halloween candy, boxes I've received as samples, ... you name it, I probably sent it. 

I wish someone sent me something like a purse!

We also care about what the box says on it if it orriginally contained something hazardous or non-mailable.  Beer cases, wine cases, etc. are great strong and would be great for mailing except for what they say on the outside.  We can't accept them because even if we know theres something else in them the people putting them on air planes don't and aren't allowed to assume so to avoid problems in transit we can't accept them.

Alright, thanks for clearing that up everyone!