One of the reasons that "Media" and "Book" rates are slower is that they go through the bulk mail center. Bulk mail consists of parcels, advertising mail, etc. No first class, priority, periodicals, or Express Mail, goes to the bulk mail center. So, if you do send a book in a manilla envelope, it will be processed with all the heavier packages in the bulk mail center. All processing is done with machinery. I would never recommend using a manilla envelope for a book. At least use a padded mailer or a cushioned mailer...