an archive of royal letters and books unseen for 200 years is to be digitised .
the queen launched project at windsor castle 's royal library .
included are letters shedding light on the american war of independence .
also a 200-year-old poetry book written by the shah of persia in 1812 .
queen joked that , ` you do n't get gifts like that any more ! '
also includes an essay by george iii and letters from queen charlotte .