The Royal Library of Ashurbanipal.
The library, rediscovered in 1850, contained about 1,200 distinct cuneiform texts spread across up to 30,000 clay tablets. Ashurbanipal, originally trained as a scribe, had a passion for learning and understanding, as evidence by his huge collection of writings on a wide variety of topics.
The library also had some of the features that we would expect today's libraries to have but would have been novel in ancient times, such as the tablets being grouped by subject (history and government, geography and science, literature and word lists, religion), those groups being in separate rooms with a catalog of that room's contents. Ashurbanipal's library even had a room for classified materials, which were buried deep beneath the palace in a separate room altogether and would have been guarded, no doubt.