.acsm can only be used with Adobe Digital Editions and stands for Adobe Content Server Manager.
For more information about Adobe Digital Editions please see http://www.adobe.com/products/digitaleditions/faq/
Side note (may not be generally applicable to Digital Editions users): Stated in the agreement (may be of concern in making a program to access .acsm ) for Adobe Content Server section 11.1 you agree not to:
(d) obtain or attempt to obtain any materials or Information through any means > not intentionally made available through the Services;
Interpret this as you will, I will not give you legalillegal advice.