The plot was worthy of a Dan Brown thriller – two Charles Darwin manuscripts worth millions of pounds reported as stolen from Cambridge University library after being missing for two decades.

The disappearance prompted a worldwide appeal with the help of the local police force and Interpol. Now, in a peculiar twist, the notebooks – one of which contains Darwin’s seminal 1837 Tree of Life Sketch – have been anonymously returned in a pink gift bag, with a typed note on an envelope wishing a happy Easter to the librarian.

The bag was left on the floor of a public area of the library outside the librarian’s office on the fourth floor of the 17-storey building on 9 March, in an area not covered by CCTV. Who left them and where they had been remains a mystery.

Dr Jessica Gardner, who became director of library services in 2017 and who reported the notebooks as stolen to police, described her joy at their return as “immense”. “My sense of relief at the notebooks’ safe return is profound and almost impossible to adequately express,” she said. “I, along with so many others, all across the world, was heartbroken to learn of their loss.

It was back in 2001 that the notebooks, which represent some of Darwin’s first inklings of his radical theory of evolution by natural selection, were originally found to be missing. They had been removed from storage to be photographed, and work was recorded as completed in November 2000. But during a subsequent routine check made in January 2001, it was found they had not been returned to their proper place. At the time staff believed they may have been mis-shelved.

Which is author’s works would you like to get your hands on?