An archive of 30 autograph letters from Arthur Conan Doyle on a real life criminal case is valued at £20,000-40,000 ($30,776-61,553) ahead of a sale at Bonhams London.

The letters relate to the Edalji affair, which saw a Birmingham solicitor of Anglo-Indian heritage named George Edalji imprisoned for animal mutilation and writing threatening letters in 1903.

Conan Doyle investigated the case of Anglo-Indian lawyer George Edalji in the early 1900s

In 1906, shortly after his release, he wrote to Conan Doyle asking for his help in securing a pardon. The author was convinced of his innocence, writing: "He was as guilty of the offence with which he was charged as was Cain of the murder of his brother."

He took up the case with as much vigour as his celebrated creation, writing reams of letters to the man in charge of the investigation - a Captain Anson. There are also letters to the vets who investigated the mutilations.

Ultimately, he fingers a farm worker named Royden Sharp, explaining: "I am convinced he was concerned in some of the earlier outrages… [he is] fiendishly cunning (with foolish intervals).

"Colour prejudice may have been enough to prompt them [the Sharp brothers] to bait the Edalji family in the cruel way they did."

Anson was unimpressed (at one point Conan Doyle was writing to him twice daily). He writes in the summation of the case: "It was on 'evidence' and 'proof' such as he obtained in the above instance that the great Sherlock Holmes based his accusations…"

Edalji was grudgingly pardoned, but the animal mutilations and poison pen letters continued for a number of years. The culprit has never been satisfactorily identified. 

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