That's a lot of books, Mel. Do you read them or did you buy them just for flattening posters?
One method that I used for flattening folded posters is to take two big pieces of cardboard and sandwich the poster in between them, put a few pieces of tape on the outside of the cardboard to prevent them from shifting, and then put the pieces of cardboard under the bed you sleep in (assuming you sleep in a bed)
, and check it every few days to make sure the cardboard has not shifted and everything is good. I did that on this beaute, a poster that was only available for Game Stop pre-orders of
The Lost and Damned, an expansion of
Grand Theft Auto IV. The method definitely flattened the poster, but it may have added some "wear" to a couple of the fold lines, so I would recommend trying this method on a poster that is not too expensive first.
Now I have an Avatar lenticular that I use to flatten out posters (since I have not bought a lightbox) and it works great on posters that are about the size of a one sheet.
I used the "Avatar lenticular" method on these beautes, a few more posters for
Grand Theft Auto IV, and there was no extra fold wear, but since the Avatar lenticular does not weigh 215 pounds, they did not get nearly as flat as the others, so I will probably use the cardboard sandwich method on these also.