It appears that it's impossible to get ACSM files from OverDrive and the Libby app now. The only way is to use libbyapp.com, but when I was trying to figure out how to get an ACSM file, I clicked the Kindle option and "locked in" that loan type. I now have to return the loan and put myself on the hold list again in order to change to a different loan type. Ah, technological progress!
