Angel of Vengeance by Douglas Preston

Preston & Child continue their #1 bestselling series featuring FBI Special Agent Pendergast and Constance Greene, as they take a final stand against New York’s deadliest serial killer: Pendergast’s own ancestor…and Constance’s greatest enemy.

A desperate bargain is broken…
Constance Greene confronts Manhattan’s most dangerous serial killer, Enoch Leng, bartering for her sister’s life – but she is betrayed and turned away empty-handed, incandescent with rage.

A clever trap is set…
Unknown to Leng, Pendergast’s brother, Diogenes, appears unexpectedly, offering to help—for mysterious reasons of his own. Disguised as a cleric, Diogenes establishes himself in New York’s notorious Five Points slum, manipulating events like a chess master, watching Leng’s every move…and awaiting his own chance to strike.

A vengeful angel will not be deterred…
Meanwhile, as Pendergast focuses on saving the unstable Constance in her fanatical quest for vengeance, she strikes out on her own: to rescue her beloved siblings from a tragic fate and take savage retribution on Leng. But Leng is one step ahead and has a surprise for them all…