The Old Library Building is a sweeping massive piece of architecture. The last photo was the central facade, and this one shows one of the two curved facades (left).
While on the roof of the central facade there's a crown, on this left facade, an eagle. 
The building was commissioned by King Frederick the Great and completed in 1780 to house the Royal Library of Prussia. It functioned as an independent royal and state institution rather than a city municipal building.
Humboldt University building is across the street. It did not have its own library when it was founded in 1810.
The Old Library building was officially handed over to the Humboldt University in 1914 after the state collection outgrew the space and moved to a larger facility.
Today, it remains part of the university campus and houses its Faculty of Law.