Ambrose Bierce, The Devil's Dictionary
After a mystery writer passes the 200-page mark, it's all ballast.
Joe Queenan, One for the Books
Yet I see reasons for all this padding and ballast, not just in Hill's mystery but in the others, as well. One is character development. In some of the shorter classic mystery novels Queenan praises, including those by Agatha Christie, Ruth Rendell and Georges Simenon, the heroes sometimes seemed to have no private lives. What did we know about Jane Marple's life when she wasn't solving mysteries or before she started solving mysteries? Not much. Alexander McCall Smith could easily tell his mystery stories in fewer than 100 pages. His novels are mostly about the personal lives of his main characters. We read his novels for the padding, not for the mysteries.
The ballast in mystery novels also serves to misdirect the reader. Not knowing until the end who the murderer is, we don't know which of the seemingly unimportant bits of the story might prove to be important after all. The killer is usually hiding in those pages and pages of details. One reason the Bill James novel was so short was that the plot involved just finding a killer, not in picking the killer out of a number of possible suspects. To do the latter in a convincing way requires a few pages, and a lot of apparent padding.
Finally, Susan Hill, like so many other contemporary mystery novelists, strives to be a "serious writer." That is, her novel purports to be something more than just a murder mystery. Hill uses her book to comment on changes occurring in the Church of England and on programs for helping prostitutes improve their lives and the lives of their families. The mystery may interest her readers, but the padding may be what most interested Hill.