A water line broke at Lexington's Central Library early Tuesday morning, causing water to leak into several floors.
Officials say the line went to the cooling tower on the roof.
Water also got into the library's Kentucky Room, where several rare books and historical items are stored.
Many of them were damaged, including the only full remaining copy of the Kentucky Gazette, published in 1787.
Some of the most heavily damaged materials will be freeze dried by a book preservation company.
Experts will now assess the damage, and determine what should be done next to restore everything to useable condition.
The Central Library will be closed all day Tuesday, but all other Lexington library branches will be open as normal.