The Diamond Conspiracy. Ministry of Peculiar Occurrences, #4

For years, the Ministry of Peculiar Occurrences has enjoyed the favor of Her Majesty the Queen. But as agents Brooks and Braun soon learn in the fourth novel in the steampunk adventure series, even the oldest loyalties can turn in a moment.Having narrowly escaped the electrifying machinations of Thomas Edison, Books and Braun are looking forward to a relaxing and possibly romantic voyage home. But when Braun's emergency signal goes off, all thoughts of recreation vanish.
Braun's street-wise team of child informants, the Ministry Seven, is in grave peril, and Books and Braun must return to England immediately. But when the intrepid agents finally arrive in London, the situation is even more dire than they imagined. The Ministry has been disavowed, and the Department of Imperial Inconveniences has been called in to decommission its agents in a most deadly fashion. The plan reeks of the Maestro's dastardly scheming.
Only, this time, he has a dangerous new ally-a duplicitous doctor whose pernicious poisons have infected the highest levels of society, reaching even the Queen herself...