The .acsm file isn't the book - it a purchase ticket that allows you to download the book which will be either a PDF or ePub file.

Install Adobe Digital Editions and double click on the .acsm file to download the book.

In Adobe Digital Editions you can find the path to the book in the "Item Info" dialog. You can bring up the "Item Info" dialog from the Reading Menu (in Reading View) or pressing Ctrl+I (Cmd+I on the Mac).