Which city's library rivaled Alexandria, built at Pergamon?

Pergamon or Pergamum, also referred to by its modern Greek form Pergamos (Πέργαμος), was a rich and powerful ancient Greek city in Aeolis. It is located 26 kilometres (16 mi) from the modern coastline of the Aegean Sea on a promontory on the north side of the river Caicus and northwest of the modern city of Bergama, Turkey.