Discovered in 1643, Codex Regius (Konungsbók in Icelandic) was thought to be written in the 13th century and contains Old Norse poems from another book, the Poetic Edda. Brynjólfur Sveinsson, the then-Bishop of Skálholt in Iceland found it and gave it to King Frederick III of Denmark as a gift (that’s why it’s called Codex Regius—it’s Latin for King’s Book or Royal Book). It was eventually return to Iceland in 1971 where it remains.
Codex Regius contains 32 texts and poems including Völuspá, which tells the story of the creation, death, and rebirth of the world. One of the poems was also read in English by Werner Herzog in his film, Into the Inferno.
You can read more about Codex Regius on Germanic Mythology.
Other codices: Codex Argenteus: the mysterious Gothic Silver Bible and Codex Seraphinianus, a weird-ass book