There's also 'Sullivan and the Unlikely Ally' which is a great read. The author does an amazing job of portraying the OC's unique personality. The interactions between the OC and Inspector Sullivan are filled with tension and chemistry. It really makes you root for their relationship as the story progresses.
You can try fanfiction websites like Archive of Our Own (AO3). It has a large collection of various fanfictions, and you're likely to find quite a few 'Inspector Sullivan x OC' ones there. Just use the search bar and type in the relevant keywords.
One interesting character is Flambeau. He was once a criminal but later becomes an ally of Father Brown. His transformation from a wrongdoer to someone on the side of good makes his character really engaging. Another character is Mrs. McCarthy. She often provides some local gossip and insights that help Father Brown in his investigations in a rather unexpected way.
Father Brown was a fantasy novel written by the world's tyrant singer. Currently, the original authorized publishing house of the novel was Reader's Press.
Some of the best Father Brown short stories include 'The Blue Cross'. It features the clever priest Father Brown outwitting a criminal through his sharp intellect and deep understanding of human nature. The story is full of twists and turns that keep readers engaged.