The Imperial Records Library was a computerized system holding the government records of the Imperial Remnant. It was moved around along with the Remnant's base of operations. It contained Emperor Palpatine's personal files, including a list of Royal Guardsmen; it was through it that Vilim Disra discovered Grodin Tierce's former status as one.
It existed as early as 9 ABY, at which point Thrawn installed a tracking device in it, to allow the Empire of the Hand to always know where the Imperial capital was. By 19 ABY, the Library had been moved to Bastion.