An acsm file is kind of like a ticket to get an e-book. After you download an acsm file, go to the Downloads folder with Finder and open the file. Adobe Digital Editions should open and download the ebook, *but* if you're leaning toward Sony, you should probably delete Adobe Digital Editions, since the Mac version doesn't support the Sony Reader.
Sony Reader Library software supports library ebooks as well as ebooks purchased from Sony and third-party stores, so you should install that instead. Once installed, open the ebook Store (from the left pane of the application) and create an account. After you're registered, you can use DRM-protected files, including library ebooks. Just click the acsm file from Finder and Reader Library will process the file and download the ebook.
When you get your new reader, you can set up Reader Library to automatic sync or manually drag/drop books to the device via USB.
Alternately, you could use Calibre to transfer books to your device, but you'll still need Reader Library or Adobe Digital Editions to turn acsm files into ebooks.
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