[Runscott]

Rhys' suggestion is your best option. Once you get it apart from the envelope you will need to let it dry, then as soon as it is dry (i.e-not completely, but appears to be dry-there will still be minute amounts of water in the paper, allowing it to be flattened), put a sheet of mylar over it (cut a sleeve apart) and stack books on top of it. You will need to check it constantly to make sure it doesn't stick to the mylar - I don't think it will (from my experience doing something similar) but I would hate to test that theory too strongly.

Rhys or others - is there something you would suggest other than mylar? That's what I used on my Burke Williams after intentionally cleaning it with water, but it's possible there is something that will actually absorb the water without sticking to the print. The mylar just protected the print from the book surface, allowing it to flatten.