The fire at the National Library of Wales was in an area referred to as 'Stack 2', where books that have been recently donated or purchased are taken.
Staff are not able to estimate what damage has been done, as they have not been able to go back inside.
Arwel Jones, Director of Public Services at the library, admitted they are "terribly worried" about the damage that may potentially have been done to the collection.
He confirmed all of the staff and readers using the library got out safely.
More top news
Show Chancellor John McDonnell has committed Labour to building the proposed Swansea Tidal Lagoon, if his party gains power at Westminster.
A man has been arrested after the display of offensive images on a Cardiff billboard.
A man and woman have appeared in court charged in connection with the death of Olympic cyclist Chris Boardman's mother.