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.
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All calm and quiet overnight, but southern areas could be in for a surprise tomorrow!
Scottish people living in Wales have been giving ITV News their opinions about the Scottish referendum vote on Friday.
While the campaign trail in Scotland has hotted up today, in Wales businesses are keeping a close eye on tomorrow's decision.