Citac. Ask why your friends are using m4b as it is only really playable on computers or itune compatible stuff. They can put their book covers into mp3's which every player can handle. This link will give you some explanation on what I mean and a solution to convert. I have no idea if it works as I touch nothing Apple. Just trying to help.
http://itunesm4ptomp3.wordpress.com/...iobook-to-mp3/