The keys to the street

Set in and around London's Regent's Park, where the city's wealthiest, poorest, kindest, and most vicious citizens all cross paths, The Keys to the Street tells of the deadly thanks a young woman risks receiving in return for an act of selfless generosity.
"Is it true that we dislike those who have done us a service?" asks Mary Jago's grandmother. One of many questions about the best and worst of human nature, it is one with an answer Mary will discover for herself as a consequence of donating her own bone marrow to save the life of a young man she doesn't know....
"It's us he's after," says Dill, "our sort." Dill's sort are the homeless who seek refuge in the park, whose corpses have lately been turning up impaled on the spiked railings that surround it....
Mary is not their sort at all and would under ordinary circumstances be separated from such horror by social barriers stronger than iron bars. But she has performed a bold act, and the circumstances of her life are now extraordinary -- she is receptive to previously undreamed of happiness, and vulnerable to the darkest grief.

"From the Trade Paperback edition."