The file extension ACSM identifies so-called Adobe Content Server message files. The files are used by Adobe Digital Editon and are a kind of key file (key) for download and authorization.
Adobe Digital Edition is a program for managing and reading ebooks and other digital publications. If you buy an e-book or an empty one (for example, from the example page of Adobe Digital Edition), you just download a fairly small ACSM file.
This file can be opened with Adobe Digital Edition, whereupon the corresponding book is downloaded. When purchasing e-books, the ACSM file also checks which rights are available for the respective book (read / copy / print). Acsm files are a kind of DRM (digital rights management)
The books themselves are available in different formats, for example EPub or pdf. With ctrl + I (Windows) or Cmd + I (Mac) you can view the information of the currently open book, including the storage path. However, it should be noted that there are protected, protected pdf files do not display with the Adobe Reader.