This is fairly easy to do, what you need to do is to navigate to the folder where you have opened your Bookshare book. And then you right click the XML file, and then click “open with” and then click “XML editor”. When this is done you just save the file as an HTML file, and now you just drop the HTML file into Calibre. Now you just click the “convert book” button in Calibre and choose your output format. Here is a YouTube video showing how this is done.
Here is a similar blog post that deals with the same problem.
Here is a YouTube video that shows how you could use TextAloud to convert Bookshare books into audio files.