I believe that is the ADE license file containing the encryption key, not the book..you are trying to open the wrong file if you are trying to open a file with the extension .acsm...find the actualy book which will be a PDF/epub or whatever.
Here are some resources which will give you the details to figure it out rather than rehashing it all here before you exhaust the info there:
https://wiki.mobileread.com/wiki/ACSM
http://califa.lib.overdrive.com/07E4...50/en/Help.htm
I am assuming you have the appropriate reader software installed, as in Adobe Reader or whatever...
EDIT: Here is the Adobe reader software you need installed to read secure ADE PDF/epub books
http://califa.lib.overdrive.com/07E4...Reader-ADE.htm