Hi Diana, It's indeed a confusing phrase! I parse it as horror writers who write stories in a "meat-market" style. Generally, when meat-market is used as an expression, it implies a place where you interact with people impersonally. I think in this case, it has the additional sense of a bloody style of story, where people are killed off with little fanfare. Thus, the sentence is saying that some people might find Uncle Silas quite violent, but if you're used to reading modern horror stories you might not think it is so violent. This further implies that the author thinks most people should consider Uncle Silas violent, but that horror stories are too violent nowadays and so many people will incorrectly see Uncle Silas as not particularly violent. Hope this helps!

My interpretation of meat-market in this context would be the more old-fashioned meaning. It’s been used to describe the genre of pulp fiction and ‘thrillers’ that are unabashedly gratuitous, full of violence and gore with no purpose but to shock the audience. So with that in mind, the sentence is saying that readers used to this form of entertainment would hardly notice any violence in Uncle Silas.