If you would rather sell your audiobook through the store on your website, there are two options. I can create a single MP3 or an M4A file. Both files will contain the book cover graphic. The advantage of M4A, is that you can display unique images for each part, chapter and section of your audiobook. This make navigation and bookmarking on your smartphone a lot easier. We can go even further by displaying important figures, tables, photos and infographics. I guess that makes M4A an audio visual book!
Here’s a quick description of the M4B/M4A format in (almost) non-tech terms: