The Book of Soyga
Dee was reportedly told by the angel Uriel that only the archangel Michael could read its tables. In 1998 a cryptographer found they were generated by a simple recursive rule — a human puzzle solved four centuries after a spirit allegedly declared it unsolvable.
Deborah Harkness's 1994 identification of the manuscript (Bodley MS 908, Sloane MS 8); Jim Reeds's 1998 decoding of the letter tables
A Latin magical treatise John Dee owned and consulted his angel about, lost after his death and rediscovered in 1994.