In this sequel to The ThreeBody Problem, Vanessa Duncan is called upon to save an innocent young woman, falsely suspected of murdering her older and unlikable husband. Although there is no mathematics directly connected to the murder this time around, by virtue of her job as a mathematics teacher and her friendships (many through her fiance) with mathematicians at Cambridge, mathematics remains a significant component of the novel.
In particular, the points of mathematical interest are:
 A discussion (pp. 19 and 99) of the fact that one of her female students is taking the exam for admission to Girton College to study mathematics. Since the novel takes place in the 1890's, this portrays an important step forward in gender equality.
 (p. 22) The history (up to the 1890's) and mystery of Fermat's Last Theorem are introduced via a character named Korneck who is trying to prove it. (Is this character supposed to be the real mathematician Korneck who in 1873 found interesting examples concerning sums of cubes?)
 (p. 24) As part of the history of FLT, we hear again the story of Sophie Germain. This story is repeated frequently in mathematical fiction, but this time some less well known details are related. For instance, although it is common to explain that she wrote under the pseudonym "M. Le Blanc" so as to hide her gender, this account explains further that there was a real student "Le Blanc" who had dropped out and it was his identity that she was using.
 Hermite is mentioned (p 160) as an excuse for the mathematicians to accompany Vanessa to Paris.
 Korneck relates a story (p 173) about a fight between Cauchy and LamÃ© concerning which of them had proved Fermat's Last Theorem first. Of course, neither of them had, as Kummer eventually points out. I had never heard this tale before, and it was interesting to me because it puts some personality and emotion behind names I frequently encounter in drier settings.
This mathematical sideline is not closely tied to the main plot, although the story of Germain helps inspire Vanessa's eventual resolution of the mystery. But, somehow it does not seem extraneous or forced. Rather, the murder mystery and the parallel story of the mathematics (as well as several romances) together form a lovely novel that nicely captures the spirit of Victorian England.
My only complaint about the book is that while the gender bending which is a key to the mystery may seem shocking and unexpected to the characters in 1890, it is nothing too surprising to me in 2006. Consequently, the clues that confound Vanessa and her eventual understanding of the situation seemed drawn out and became tiresome to me.
Other than that, it was a pleasure to read, does a very nice job of presenting real mathematics in an a fictional context, and provided me with quite a bit of mathematical history that I had not previously known.
The author, a mathematician who chooses to remain anonymous, is a professional mathematician. I know this in part from direct (but still anonymous) correspondence, but another clue is a misprint in the mathematical notes at the end of the novel. It says "...for the exponent $n=3$, but there..." As any modern mathematician would recognize, the dollarsigns there are intended for the mathematical typesetting software "TeX". It is funny to see that it ended up being typeset literally into the book.
