It might be easy to understand why I enjoy reading Stephanie Burgis's combination of real 18-19th century history, romantic adventure, and touches of magic. She has an impressively solid familiarity with the history and manners of the era she draws from (which, if you check out the topics of her graduate education, is no surprise). The Congress of Vienna, sorting out the political consequences of Napoleon's defeat, is a natural setting for intrigues of all sorts.
Two people, neither of whom is the person they current portray, encounter each other in the build-up to the Congress for the first time since a violent separation when they were children. Michael, once apprenticed to a political pamphlet printer, has survived by learning the arts of the con man and has arrived as the disenfranchised Prince Kalishnikov, hoping to restore control of the realm Napoleon stole from him--or at least to convince someone to pay him off to go away. Karolina, the daughter of that printer, fell into the hands of the head of the Austrian secret police, who maintains his power by alchemical rituals that drain energy from his victims. Handed off to be the plaything of an English aristocrat, she turned her situation around and became the (now widowed) Countess of Wyndham. Her goal at the Congress is to free her father from the secret prison where he's been held for decades and, if possible, to avenge herself on those who held him there. Their accidental reunion in Vienna could spell disaster for both their plans--or each just might have found the only ally that could ensure success.
I enjoyed the casual details of the setting and historic personalities, as well as the solid back-story for the central political tensions. The rich diversity of early 19th century Vienna came alive on the page. And if I occasionally felt that certain bits of the historic background were being repeated more often than I needed, keep in mind that I'm on the far end of the scale of "just give me a hint and I'll be fine," as well as being a bit more grounded in the historic outlines than the typical reader.
The interpersonal interactions driving the plot worked very well for me for the first three-quarters of the book, including the completely expected growing romantic tension between the two protagonists. Very much in the genre of "I'm totally attracted to you but I can't trust anyone--and especially not you--so giving in to it would be a fatal mistake." The romantic tension was only slightly spoiled by a few too many (in my opinion, unnecessary) incidents of "I saw you smiling at so-and-so, which means you're actually going to bed with them, so my heart is broken, not that I'll admit that I cared."
But in the climax of the book, my suspension of disbelief slipped a little. Too many key players were too easily convinced, too quickly, to believe the protagonists' stories in the nick of time, and to pitch in at the risk of their own lives and careers, or to back down from opposing them far too readily. I had anticipated the fate of one key character from the very beginning--a fate that it was essential for the protagonists never to consider seriously. The strongest point in the climax was our heroine contributing actively and believably to her own rescue, rather than becoming a damsel.
Congress of Secrets is a fun romantic adventure, with a solid grounding in history and a reasonably satisfying conclusion. It is very loosely connected to Masks and Shadows, set in the previous generation, and there are a couple of Easter Egg references to characters from that book, but the two can be read entirely independently.