I might be rolling back to the previous update. (I still have other formats to check out)
Synopsis:
With mp3 music playing (and Volume control working on Jog Wheel).
I went and resumed reading my 500K EPUB book.
2 minutes 
to get loaded and show my last place!
During that time,
Music went silent (I am trying to avoid using names stop and pause that are found on controls, as this was not controlled.), or stuttered with 10 second gaps.
Subsequent page turns were intermittently snail slow, with more music interruptions.
Pressing the Menu button; about 15 seconds elapsed before any response (and the music went silent).
History:
Except for the lack of external volume on early updated firmware, the music played flawlessly, during book loads, page turns
and with little impact on file load and resume.
NB: The same EPUB takes about 20 seconds to resume without music playing, which is close to the previous FW time.
The Good:
Nine, well labeled Zoom levels for EPUB