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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It should continue to become drier and brighter from the west. However, a sharp shower is possible.
The heaviest bursts are expected in the south and east, with some western areas either remaining dry or becoming dry by dawn.