A small part of the collection at the National Library of Wales in Aberystwyth is thought to have suffered water damage following a roof fire.
Arwel Jones, director of public services at the library, said water used to fight the flames on Friday had leaked through five or six floors.
An investigation has been launched after the blaze destroyed a section of roof on what is known as building two.
It took around 50 firefighters nearly four hours to bring it under control.
A specialist salvage company from Oxford has been assessing the damage caused to the collection.
The building was closed to the public on Saturday and will remain shut on Monday, but it could reopen on Tuesday.
Up to 300 staff and 100 visitors were evacuated when the alarm was raised at 14:40 BST on Friday.
The affected area was largely used as office space and for new acquisitions.
"It's been a very, very busy day. The specialist retrieval staff have arrived from Oxford - they've been here all day on site," said Mr Jones told BBC Wales on Saturday.
"The fire brigade have been here all day finishing off their work and conducting their investigation and the library staff have worked very, very hard on all sorts of tasks.
"All that water being poured into the top of the building has literally poured through five or six floors, so there's water damage all through that part of the building.
"That's damage to the structure of the building and to the offices and, of course, any collections that were being worked on at the time.
"As far as we can ascertain at the moment the damage to the collections is limited. This isn't storage facility - it is office space - so it could be much, much worse than it is."Part of library collection damaged
Mr Jones said the library would be open to staff on Monday and around 70 would be relocated after their offices were damaged by the fire.
Workers had been repairing the roof near to where the fire started, but the library said it was "not in a position to draw any conclusions from this".
Reports from the scene suggested a 100ft (30m) section was destroyed.
Mr Jones described the library's significant collection as the "memory of the nation".
He said it contained six million books, a million photographs, 30,000 manuscripts, works of art and maps.
Many of the library's books were unique and would not be found elsewhere, he added.
Firefighters, some from as far afield as Carmarthen 50 miles (80km) away, attended the scene along with 11 fire appliances, and it was not until 18:30 BST on Friday that the blaze was put out.
Library staff said it was not known how the fire started, but everyone was safe.
Aled Gruffydd Jones, who takes over as head librarian in August, said on Friday that he was puzzled by how the fire could have started. "Clearly we need an inquiry," he added.
The Aberystwyth University sports centre, which is next door to the library, was closed as a precaution.