"LAIR OF THE LIBERTINES" (Third Season)
Father Brown finds the book among the various other works of fiction which the killer is going to take with her when she makes her escape. Included in the pile are various editions of German erotica, "Business As Usual", "With Virtue", and the key to figuring out the killer's true identity, "African Safari".
"CRACKPOT OF THE EMPIRE" (Fourth Season)
While keeping an eye on Mrs. McCarthy after she was poisoned, Lady Felicia started reading the novel and was quite aroused by the experience. (See the picture at the top of the page.)
"THE HAND OF LUCIA" (Fifth Season)
This is the episode which brings the book front and center in the investigation as it is key to the solution of the murder. I won't say more so as not to spoil it, but I really don't know much more than the identity of the victim. I have yet to see it.
Whether or not the book shows up in other episodes, I have yet to find out. (I'm only up to the middle of the fourth season. I had been watching the series via CPTV, but now I find the first five seasons are available on Netflix so I'm up for some binge-watching!)
But I think "Lulu and Lucia" is a strong candidate for the 2017 Toobits Award for Best Fictional book in Toobworld, even if it is to be found only in an alternate TV dimension. But who knows? Maybe one day another TV show might reveal that it can be found in some TV character's collection.....