Anna and Christian become instant friends due to a quote from Monsieur Lecoq. Most likely only very few have heard of him now, but Emile Gaboriau wrote before Conan Doyle.
Monsieur Lecoq first appears in the novel ‘L’Affaire Lerouge’, which was published as a serial and novel in 1866 (in its 9th edition by 1870) and can lay claim to be the first true detective novel. Unlike Holmes, Lecoq is part of the police force of Paris and follows standard investigative methods, described very realistically.
Lecoq is even mentioned in ‘A Study in Scarlet’:
‘Have you read Gaboriau’s works?’ I asked. ‘Does Lecoq come up to your idea of a detective?’ Sherlock Holmes sniffed sardonically. ‘Lecoq was a miserable bungler,’ he said in an angry voice. ‘he had only one thing to recommend him, and that was his energy. That book made me positively ill. (…) It might be made a text-book for detectives to teach them what to avoid.’ So, not impressed.
Regardless of Conan Doyle/Sherlock Holmes’ opinion of Lecoq, Gaboriau became rich from his writing, partly because many newspapers published his stories as serials. He also had a ‘hobby’: observing passers-by and guessing their profession, social status, etc., based on their appearance, and then shadowing them to see if he was right. Does that remind you of anyone?
Gaboriau also became popular in Denmark, and in February 1869, Dagens Nyheder announced that they would continue serialising his novels. The last one, ‘Falsk Spil’, ran in 1870. Gaboriau died in 1873. By 1910, he was pretty much forgotten about – everything was about Sherlock Holmes.
I’ve read ‘Monsieur Lecoq’ in English. It’s ok.
(There might be detective novels, I have never heard about.)