Crane plays MP3, M4B, M4A, FLAC, AAC, and OGG.
Embedded chapters and cover art are read automatically out of your files, so a well-tagged M4B lands in your library already looking right.
Crane plays audiobook files you already own. There is no store inside the app — you point it at your folders and it does the rest.
Got a file that will not import? Post in Bug Reports with the format and roughly how long it is.