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You just need to keep backups of the original books.
If I understand the standard correctly, then the client is supposed to update the license definition file included in the eBook whenever it receives a status update (like a revocation) from the server. So if you are able to open a particular book right now, and you make a backup of that book file, you'll always be able to open that book even if the license gets revoked (as long as you know your account password). The reader might update (and invalidate) the copy that it has, but if that happens all you'd have to do is disconnect the reader from the internet, replace the revoked book file with the book file from your backup, and then you'll be able to open it again.
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