The Codex Calixtinus is a twelfth-century illuminated (illustrated) manuscript with a collection of works related to Saint James and the pilgrimage to Santiago de Compostela. It includes sermons, music, stories about miracles performed by Saint James, and the earliest guidebook to the pilgrimage routes from France.
The best-conserved version of the manuscript (not the only copy), was kept at the Santiago Cathedral.
It’s not there any more. Its absence was first noticed on July 5, 2011, but it may have been stolen any time in the previous week. The manuscript, described as “priceless,” was not insured, since the insurance cost had been estimated at six million euros.
You can read more about the robbery in the Guardian and The Olive Press. There’s also a more recent article from El Correo Gallego in Spanish, that describes how researchers generally used a photographic reproduction of the text, and only consulted the original when they couldn’t make out a subtle but historically and linguistically important detail.
To learn more about the manuscript itself, there’s a helpful Wikipedia article. You can also read the Pilgrim’s Guide section of the manuscript on-line.