Weighed in the Balance (William Monk Mystery, Book 7)

It's 1859 and throughout Europe tremendous upheavals have taken place. Hester Latterly is nurse to the sick son of a German Baron and his family, who have moved to London from one of the many small principalities between Prussia and Bavaria - and the Baroness tells Hester about her kingdom's famous royal family...
Handsome Prince Friedrich was one of just two heirs to the crown, considered the perfect match by every woman of the land. But during an affair with Countess Zorah Rostova, he meets the alluring and sophisticated Gisela - with whom he falls deeply in love. He can have Gisela or the crown, but not both. He chooses Gisela, marries her in Venice and, after many years, tragically dies in England.
Now, Countess Zorah, having accused the widowed Princess of murdering Friedrich, is being sued in the biggest slander trial of the century - and the only way that she can defend herself is to prove that Gisela is indeed guilty. But in doing so she must sully the greatest love story that the country has ever known, and that is enough to put her lawyer's career in jeopardy, too. That lawyer is Oliver Rathbone, who calls on Investigator William Monk to help...