I'll say what I can about this book, but I think the best review is Stephen King's in The New York Times of September 27, 2016, and I would recommend it highly.
Lib Wright is an English nurse, recently returned from the Crimean War, where she was trained by Florence Nightingale. In flashbacks we see the toll the nursing took on Lib, but also her pride in her scientific, detached method of nursing. Wright is hired by a town committee in a small Irish town to watch over Anna O'Donnell, a young girl who has eaten nothing since her 11th birthday--almost four months without any food. In an afterword, Donoghue explains that O'Donnell is based on at least 50 cases over the last 400 years of "Fasting Girls," but King, in his review has done some additional digging and says that O'Donnell's is a "case that most closely resembles...that of Sarah Jacob, a Welsh child of 12 who was said to have gone without food for more than two years. After her story was reported, a team of nurses was hired to keep watch and discover if the girl really was fasting."
In The Wonder, Wright discovers an Ireland that has been decimated by the Great Famine, and a country that is practically ruled by Catholicism. It becomes clear to Wright that the town committee, for the most part, truly believes that Wright, and her partner nurse, an Irish nun whom she hardly trusts for accurate reporting, will find that Anna is a saint, living without food. Wright, on the other hand, begins as a cynic, assuming the family is doing it for the money or the notoriety, and below that is a general English disdain for the Irish, seeing them as backwards and uncivilized (which Wright tended to bang on about so constantly that it cost at least half a star). When she learns that the family is truly donating the money and gifts that are left for Anna, and that they seem to be pious, yet honest people, she begins to question herself, and what role her observation may be playing in Anna's fasting.
I can't go much further into the plot without spoilers, but it is a story worth reading, for the insight into Nightingale's methods, the horrors of the Crimean War, the extent of the hold the Catholic church had over Ireland at the time, and the aftermath of the Great Famine. And all of that is aside from the fascinating story of Wright's watch over a child--a job she thought of as simple--that becomes one of the most complex things she's ever faced. Another half a star lost for what felt to me like a somewhat hokey ending.