In the novel The Sittaford Mystery, Mr Rycroft is the resident of cottage No. 3 at Sittaford. A naturalist who specialises in ornithology, he is said to often out doors roaming over the moors pursuing his interest. His cottage has a fine library, and is "nearly all bookcases".
In fact Rycroft also has an interest, if not a greater interest, in criminology. When Emily Trefusis comes to Sittaford to investigate the murder of Captain Trevelyan, he offers his help. As Emily is trying to clear James Pearson, Rycroft opines that an alternative would be someone who knew Pearson had a quarrel with Trevelyan that day and took advantage of that to kill Trevelyan for his own reason knowing that the suspicion would fall on Pearson. This implied someone local, who lived in Exhampton and familiar with what went on in the house. Rycroft suggested the manservant Robert Henry Evans as a possibility.
On the seance, Rycroft said this relates to the third of his hobbies. He is a believer in spiritualism but he has no psychic powers--he is only a deeply interested observer. He is a member of the Society of Psychical Research. He found the seance very strange and interesting. "A well authenticated and amazing case. Five people present, none of whom could have the least idea or suspicion that Captain Trevelyan was murdered." He told Emily he had written up an account and sent it to the society.
Rycroft is described as a dried up little gentleman. His cottage is indeed all filled with books. Emily observed that "One section dealt with occult phenomena, another with modern detective fiction, but nd fby far the greater part of the bookcases was given up to criminology and to the world's famous trials. Books on ornithology held a comparatively small position."
Towards the end of the book, Inspector Narracott verifies that Rycroft is the uncle of Martin Dering. As Dering stood to gain from the will of Trevelyan, Rycroft was in some ways an interested party in the crime. However not much is made of this point.