I think the mechanism of encryption is a bit different than you describe. There is one encryption key for each differnt book (same content = same key). What the Adobe server does, when ADE downloads the book, is to insert the key for the book in it. Since it would be stupid to just give everyone the key without protection, the key itself is encrypted with your personal key.
Adobe puts encrypted keys for all your registered devices in the encrypted book and the registered ADEs then are able to first decrypt the key and afterwards decrypt the book.
The ACSM is only a link, all important information for the encryption of the key and the information which keys have to be inserted in the book come from ADE.