ACSM
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An ACSM file is a message file served by the Adobe Content Server to Adobe Digital Editions, a program used to manage and read eBooks and other digital publications. It contains data to activate and download an eBook. It does not include the eBook itself.
I am keeping my eyes on BentonEdmondson/knock: Convert ACSM files to PDFs/EPUBs with one command on Linux.