I was assured when I bought my nook that it could be used to listen to audiobooks as well as music. Audible.com is the most ubiquitous source of such books, but I'm finding that the file type is one not recognized by the nook. Various attempts at converting the files to mP3 have failed - both in the process of downloading the books from audible.com and also through a free converter program which claimed it could convert the files from ".aa" to ".mP3." I mostly listen to them on my ipod but this was supposed to be a handy way to take both types of books with me. Help?