From the website
This digital facsimile provides reproductions of all 157 miniatures (and facing text pages) from the Hours of Catherine of Cleves. The original one-volume prayer book had been taken apart in the nineteenth century; the leaves were shuffled and then rebound into two confusing volumes. This presentation offers the miniatures in their original, fifteenth-century sequence.
The Hours of Catherine of Cleves is the greatest Dutch illuminated manuscript in the world. Its 157 miniatures are by the gifted Master of Catherine of Cleves (active ca. 1435-60), who is named after this book. The Master of Catherine of Cleves is considered the finest and most original illuminator of the medieval northern Netherlands, and this manuscript is his masterpiece.
For other illuminated manuscript collections online, see The Pages from the Past, Central Asian Miniature and LUSAMUT Studio's Armenian Miniatures. You can see a few more pages from Books of Hours in this RIT collection to give you some idea of just how impressive this manuscript was. [via MeFi]