Great read, but there is some violence and some veiled sexual content to be aware of.
Sherlock Holmes is working on a case when another, bigger, more terrible one presents itself. John Watson faithfully follows his friend through the murky shadows of London to dig through a mountain of secrets and lies on their mission to avenge a boy and discover the elusive "house of silk".
Holmes and Watson have pretty high morals that they stick to; these are echoed by minor characters such as Lestrade, Mycroft, and Mary Watson. Criminals are, obviously, at the other end of the scale. In essence, good and bad are clearly defined.
Holmes and Watson enter a school for street boys run by a reverend. A silver cross hangs on the wall. The boys receive a "Christian education", including a divinity class (taught by the reverend's wife); Holmes and Watson walk in on a class in which the Bible is being read.
Holmes "prays" that they're not too late at one point and later blurts a "thank heavens", but it's unlikely he is a believer. Similarly, one man says, "God help her"; Watson uses a similar phrase as well as an exclamation of "for God's sake". One crime is referred to (and rightly so) as a sin.
A mentioned gang member was nicknamed "The Ghost" and was apparently feared as much as a supernatural being. A brief allusion is made to the beheaded ghosts that supposedly haunt the Tower of London.
Watson has his palm read by a fortune-teller though he's a skeptic and still puts no stock in the event.
Note that while there are several mentions of blood, it's not gory.
Several people are shot and killed. Some of these are merely mentioned in a narrative; but one man definitely receives a head-shot. It's passively mentioned that one man falls down some stairs and dies; it's briefly mentioned that another commits suicide. One man dies in an accident. A kid's body is found with multiple broken bones and a vicious cutting of the throat.
Watson mentions his injuries from the war a couple times. He's attacked with a knife (not a serious wound), threatened with guns, painfully jarred in a rough ride, struck a couple times in a fight, and carries his revolver. He shoots with intent to kill once.
A couple attempts are made on Holmes' life, including poison and bullets. He's also manhandled. At one point, he's the prime suspect in a shooting.
Mention is made of a woman who died of breathing gas fumes; another character suspects she's being poisoned.
Drug and Alcohol Content
Holmes is one of many who smoke.
Alcohol is served both at meals and on its own, a couple times for more or less medicinal purposes. Only a few background extras are ever drunk. Watson catches Holmes glancing at the case that holds his syringe and cocaine, though Holmes never uses it in the course of this story. Holmes is drugged at one point. The two enter an opium den, but not to indulge; three opium addicts are seen inside and another man admits to his extreme addiction.
Nothing explicit; it's all implied. Mention of "shabby men and gaudy women" who need shadows to do their business, which "carried shadows of its own". Child prostitution is implied a couple times. Holmes and Watson see a man wearing only a shirt.
Crude or Profane Language or Content
A man describes a situation as "d**nably difficult" and another declares a night "d**ned cold". A kid uses the British cuss word "bloody".
This is a very well thought-out story that, at least in some parts more than others, echoes the style of the Sherlock Holmes stories I'm familiar with. The mysteries (for there are actually two) are fairly thick and involved, such as we've come to expect of Holmes.
However, Watson's synopsis of the case is that there are some evils (such as a couple things he witnesses) no one should know. Said evils shake Holmes and Watson to the core and may reach out to readers beyond the level of sympathy and give them pause. While nothing is graphic, a couple elements may very much disagree with readers and/or shock them.