[6charlong]

I think of the .ACSM files as a kind of sales receipt. It is proof that you bought and paid for a copyrighted book and it contains identifying information directed at you, personally, hence proof of purchase.

I made a folder called ACSM and copy all my ACSM files to it, then change the file name to <title of book>.ACSM so I can find them later. This way I have the book backed up without having to remember which bookstore I bought it from.

This doesn't work with books from Barnes and Noble because they use a different sort of DRM without using ACSM files. Also Apple books. The Harry Potter books already have the purchaser information included in the book itself so no problems.

Kobo backs up your books--you can go and download the ePub version again if necessary, but the ACSM files work as backups too. My hard drive crashed last spring and I had to redownload several books I wanted to keep close at hand including some from bookstores that have gone out of business since I bought my books. I double clicked the ACSM files for those books and there they were.