Difference between MSC and USB

Please advise me of the difference in the settings of USB and MSC.

I am trying to download audio books and am having a number of problems.

Reader? Like … BOOK READER?

hahaha i crack myself up!!

MSC is for unlicensed content.  Meaning MP3s and WMAs without copy protection

MTP aka Auto Detect is for licensed music or subscription music.  This is probably the mode you want for the audio books, since almost all audiobooks are copy protected.