Casket of Souls by Lynn Flewelling, 2012.
This is the sixth installment in Flewelling's Nightrunner series, starring a pair of young men who are thieves and spies for the Queen of Skala. As the book opens, Alec and Seregil have returned from their adventures in The White Road and are getting back into the swing of their usual mischief: stealing letters from nobles' houses, spying on people, and uncovering wide-ranging conspiracies to kill the Queen and her heir, and another to kill the Queen's very popular youngest sister.
Just another week in Alec and Seregil's life.
The Queen has unpopularly decided to continue fighting the war by rejecting the Overlord of Plenimar's treaty. Back in Rhiminee, Skala's capital, food and materials are scarce, and the people are growing restless. Their anger only increases when a new plague of completely unknown origin breaks out and a quarantine is declared.
Meanwhile, a theater troupe displaced from Mycena, where Skala and Plenimar tend to wage their wars, has arrived in the city and their chief actor is winning hearts and minds--and many little gifts from patrons.
This is the most complex book in the series yet. Flewelling interweaves multiple plot threads: Beka, Princess Klia, and Queen Phoria out at the front; Alec and Seregil in Skala; and Atre and his theater troupe. The Skala plot includes two rival cabals and the mysterious sleeping plague, and Beka and Klia have ties to one of the cabals.
Even so, this book is likely to be readable as a standalone story. A new reader won't know who Nysander is, or understand Seregil's complex relationship with his father back in Aurenen, but the new reader should be able to recognize those as references to past books, and, with any luck, want to go read the rest.
Flewelling is currently working on what she says will be the last Skala/nightrunners book. I look forward to reading it and seeing where she goes after it's finished.