Retrieving Old Mag Issues from Newstand iPad
As someone says above, iExplorer is a good way to view the data files on the device, but my question is this - don't you have access to your purchased periodicals even if you delete them?

Here's my test case. I bought a new device, and reinstalled a few apps from scratch, including all of my subscriptions. I checked each of them, logged in with my Apple ID, and every one that I have bought has a Download icon beside it. I even have some magazines with non-consecutive subscriptions, and the editions from each subscription are available for download. (i.e., AUG-NOV 2011 and SEP-DEC 2012).