All of the mentioned novels are set in historical environments. "The Plague Lord" is set in ancient China (during the rule of Kublai Khan) and features Marco Polo as a main character. "The Rose Demon" is set during the time of the crusades and "The Soul Slayer" during the rule of Elisabeth I. "The Haunting" finally is set roughly in the Victorian period. But the historical periods aren't the reason why I like those two novels better than the other two. It's the stories they tell.
"The Plague Lord" features Marco Polo living in China. There are quite some historians who these days doubt he ever reached this empire, but then, that's not important for a novel, because it's not a scientific book anyway. Incidentally another thing the book implies is today considered a possibility: That the plague started somewhere in China and came to Europe via Venice. As Venice then was one of the most important harbours for everything coming from the Far East (via the dessert on caravans to Asia Minor and from there with ships to Venice, then the harbour doing most business with Asian countries), if the plague has come from Asia, it came landed in Venice.
But the "Plague Lord" featured in the story is not a virus. It is an entity that existed before, trapped in a tomb somewhere in the middle of China, acting (just as in real history) through a new kind of rat, a black rat, not a grey one - a plague rat. During the story it is freed, it comes to the forbidden city, to one of the largest capitals of the world, and only a handful of people can stop it (as is usual in horror stories). Of course, as always, there's also human agents, people who freely help this entity. "The Plague Lord" - pretty much like the other demons in Doherty's stories, jumps
from body to body, using human and animal minds to reach its goal.
And just killing all its human helpers doesn't completely stop it ... the end contains a hint that the rats have made it to Venice (as we, the modern readers, know - the plague has, after all, come to Europe in the Middle Ages).
Doherty, and that's obvious in all the stories by him I've read, is extremely good at picturing the historical period his stories are set in. So, while reading an interesting and sometimes quite scary novel, people actually learn something about history.
The work of the human agents - which also brings Marco Polo as a trusted acquaintance of Kublai Khan in - starts with killing those responsible for taking away the junk and cleaning the streets. Clean streets mean less rats and less rats mean less chances of the plague spreading among the humans. As China at that time had a more efficient waste disposal system than Europe, it was necessary to sabotage it for the "Plague Lord" to rule again. Marco Polo is ordered to find out who killed those men and why and this makes him cross paths with the "Plague Lord's" helpers - and after a while with the "Plague Lord" himself. There's scary parts in it, ghosts coming back to haunt Marco Polo and his household are the scene I can recall most easily, even after more than one year since the last read.
There's ages in history between "The Plague Lord" and the second novel I want to write about, "The Haunting". As the name suggests, the novel is about a haunted mansion somewhere in Victorian England. Main character (at least among the living) is a rather shy and withdrawn Catholic priest whose biggest talent is exorcism. The story starts off with him banning a ghost in a flat somewhere in central London. But central London is not a haunted mansion somewhere in the country.
His adversary is the first lady of the mansion, a dangerous and deeply evil woman who does not want to move on, who has anchored herself deeply in that mansion (in which she was murdered by her stepdaughter after a true reign of horror) and taken over the minds of various other ladies of the mansion (and is attempting it again).
Slowly the young priest and his sister unravel the story, finding clues end even, finally, the remains of the woman - necessary to drive her from the mansion completely. But the lady fights back, bringing up the ghosts we all have in our past, creating horrible nightmares and nightly haunting (thus the title).
In the end, the lady is banished for the moment, but - just as the ghost he drove out of that flat in London - she might come back some day. Some evil, that seems to be the basic line of the story, cannot be eradicated.
Again Doherty writes a masterful story in this book. He depicts the period, the people and the place - which this time is a lot closer to us than ancient China - in a very vivid way that makes it easy to imagine them. And as you can see the huge mansion with its long stone halls and large rooms in your mind, it's easy to imagine the "haunting" part with words being written in blood, a man falling down the stairs to the crypt, another man being hung in a large tree by his own scarf and so on.
Both novels - and the others as well - are very well written and can surely cool you down on a hot summer day.