The ebook I downloaded is not a PDF or EPUB file, but an ACSM file. What is it, and what software should I use to open it?

The .acsm file is not the ebook file: it's a link that your e-reading app will use to download the ebook from the platform.

To open the file, use one of the e-reading apps recommended for your device. For more information, refer to the Startup Guide for your device.