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Weighed in the Balance is the 7th in the William Monk series by Anne Perry. It has dropped to $1.99 at Kindle and Kobo US.

Kindle US: https://www.amazon.com/Weighed-Balan...dp/B0043M4Z1G/
Kobo US: https://www.kobo.com/us/en/ebook/wei...-the-balance-4

Spoiler:

When Countess Zorah Rostova asks London barrister Sir Oliver Rathbone to defend her against a charge of slander, he is astonished to find himself accepting. For without a shred of evidence, the countess has publicly insisted that the onetime ruler of her small German principality was murdered by his wife, the woman who was responsible for the prince’s exile to Venice twenty years before. Private investigator William Monk and his friend Hester Latterly journey to the City of Water in an attempt verify the countess’s claims, and though the two manage to establish that the prince was indeed murdered, as events unfold the likeliest suspect seems to be Countess Zorah herself.