He assembled at Ninevah "the first systematically collected library." A library, in Oppenheim's view, apparently, was distinct from an archive: earlier repositories of documents had accumulated passively, in the course of administrative routine. Tablets from the library of Ninevah preserve the most complete source for the Sumerian/Babylonian Epic of Gilgamesh. Other sets of tablets offer what is essentially a Sumerian/Akkadian dictionary. There are arcane astronomical/astrological texts. By far the largest group of tablets (almost all of which are in the British Museum, London) however, are 'omen' texts that taught the scribes how to recognize the significance of portents.
Ashurbanipal's reign was marked by incessant warfare. He began by making war on his brother, who had been installed as king in Babylon; Ashurbanipal conquered. Oppenheim notes the curious break in all documentation during the final decade of his rule.
Oppenheim's dates for Ashurbanipal (as given here) are drawn from references in the inscription of the mother of Nabonidus.
A. Leo Oppenheim, Ancient Mesopotamia: portrait of a dead civilization.Reference