It was founded in 1572 by Duke Julius in the castle. Under the learned and bibliophilic Duke August, it was considered to be Europe's largest collection of books and the eighth wonder of the world. Today library was built between 1883 and 1887 in the style of a Florentine palace and was to replace the dilapidated rotunda of the 18th century.
Today, the library houses a modern research library of international reputation with its 800.000 volumes, 350.000 of which date back to the 15th - 18th centuries.
Some of the rooms such as August's Vestibule, the Treasure Chamber, the Globes' Room, the Cabinet and the Illustrated Books' Room are used as a museum and can be visited. Changing special exhibitions always make a visit a very interesting event. The most precious treasure which attracts a lot of visitors every year is, without any doubt, the Henry the Lion's gospel book which was purchased by auction in 1983 for the fabulous amount of 17 million Euro. It is one of the most precious manuscripts of the Middle Ages and of the history of art, which has found its proper place in Wolfenbüttel.