It was just at the start of the microfilm boom, so they used microfilm. The quoted lifetime is 500 years, accelerated ageing tests indicate 1000 years. So the books probably wont be readable. The Egyptians did better with papyrus and ink.
A backup metal film system resides in the crypt as well.[1][2]
[1]"The New Georgia Encyclopedia — Crypt of Civilization". Retrieved 2008-06-29. [2]"History of the Crypt of Civilization". Retrieved 2008-06-29.