Angel of vengeance / Douglas Preston & Lincoln Child.

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...