desertgrandma is correct on all points. the acsm file isn't the book. It is a key file that gives you permission to open the book
1. Install Adobe Digital Editions and set up an account
http://www.adobe.com/products/digitaleditions/
When you "check-out" a book from the library and click download, tell your computer to open the acsm with Adobe Digital Editions. That will put the eBook on your computer
2. Install Calibre
http://calibre-ebook.com
3. Google the phrase "Apprentice Alf Blog" and look for the entry on "Continuing the Dark Reversers Work.
There is a set of tools there to download. Do so and look for the instructions.