The 15th-century chapel in Midlothian became world famous after featuring at the centre of a conspiracy theory in Dan Brown’s book The Da Vinci Code.
The book, published in 2003, sold more than 80 million copies and inspired a Hollywood film starring Tom Hanks and Audrey Tautou.
Thousands of tourists visit the ornate chapel after tales it houses the Holy Grail, or the skull of John the Baptist, the final resting place of Mary Magdalene or a dozen armed Knights Templar.
Historian Ashley Cowie said he found no evidence for the Holy Grail, but he said that ancient books rescued from Rosslyn Castle during a fire in 1452 could indeed be hidden there.
The castle was a scriptorium, where friars wrote manuscripts, and is thought to have been connected to the chapel by tunnels.
The blaze is said to have destroyed precious literary works, but many were saved by the Earl.
Speaking in the final part of BBC Alba documentary series Great Estates later this month, Cowie said: “There’s a legend he had three ladies in waiting and his wife, and he raced past them through the fire to get to the books. He threw two chests out the window as his servants were escorting his family out.
“There were some seriously valuable papers and books.
“Could they be under the church? Yes indeed they could. However, I like to think they are underneath the castle. That’s where the scriptorium was.”
Rosslyn has been in the St Clair family for 600 years.
Lady Helen Rosslyn, wife of the present Earl, Peter St Clair-Erskine, tells the programme: “There was a fire at the castle and the books were saved, but I don’t know where they have gone now. That may be the treasure.”
Mr Cowie debunks claims the Holy Grail is buried under the spectacular carved Apprentice Pillar within Rosslyn Chapel.
He said: “I’ve seen many pillars, and many cathedrals around the world and I’ve never seen anything so ornate. It’s craft skills beyond anything else in Scotland.
“A lot of people reckon the Holy Grail is under there, and a lot of people say the head of John the Baptist is under there. I’ve been involved in scans of that pillar and there is nothing inside.
“The beauty and the mystery is on the outside in plain sight.”
He also believes any knights buried under the chapel floor are brave Scots, and not Knights Templar.
He added: “There are most definitely knights under Rosslyn. Sir Walter Scott talked about the knights in their armour with all their weaponry around them.
“There’s lots of historical evidence to suggest that they were there, but what’s wrong with a Scottish knight? That’s what’s under there in my eyes.”
• Great Estates, is a four-part series on BBC Alba, Thursdays, at 10.30pm.