Execution Dock (William Monk Mystery, Book 16) - Anne Perry

Once again, Inspector William Monk, now of the Thames River Police, must face a dangerous foe. It's 1864, and after a game of cat and mouse, Monk has captured Jericho Phillips, the man he suspects of brutally killing a young mudlark and running an evil child prostitution ring. In bringing Phillips to justice, Monk hopes to close down the ring and avenge the memory of Durban, his old commander, who was determined to capture Philips. However at trial justice does not prevail. Oliver Rathbone, Monk's friend, is hired anonymously to represent the accused and when he proves that vital evidence is missing, Phillips is freed. As Monk begins the investigation again, venturing deeper into London's murky underworld, he realises that Durban may have had his own reasons for pursuing Phillips, and shockingly, that secret support for Phillips may reach further into civilised society than anyone could ever have imagined...