Dead March, by Ann McMillan

Here's the description from Amazon:

Narcissa Powers, a young Virginia widow consigned to a dismal existence in the country home of her sister-in-law, receives an urgent summons from family friends of her beloved brother, Charley.

Shortly after she rushes to his side, he dies of a disease that should have caused only a minor infection. The mystery of his death is compounded when Narcissa finds a fragment of a half-burned letter from Charley that someone has hidden in her Bible. Wanting to do right by her brother and avoid returning to the doldrums of her country existence, Narcissa plunges into the turmoil of Richmond in the days between Fort Sumter and the first great battle of the Civil War.

A colorful collection of plausible characters gather in the parlors and back alleys of Richmond--a British journalist, a dashing but arrogant young doctor, a cruel overseer, and Judah Daniel, a freedwoman who is also the local herbalist and "conjure woman." Each will be a part of the eventual unraveling of the mystery.

Sample paragraphs

"Mrs. Powers, I am so glad that you have come. Your dear brother...I hope that he will know you...I pray that your visit will do him some good." Mrs. Hughes seemed to be near tears. Narcissa felt the shock of her words and leaned for a moment on her supporting arm.

They went in through the open door. Narcissa had a vague impression of the inside of the house as elegant, formal, with dark, polished wood and rich, red-toned brocades. Mrs. Hughes and her maidservant were thoughtful, quickly and quietly helping Narcissa remove her traveling clothes and seeing to her luggage. She was led into the other half of the double house, then down a hallway tiled with squares of rose and gray stone, uncarpeted. They passed a book-lined office, then came to a door that stood ajar.

Rachel Hughes stopped. Rather than rudely pull away from the woman's grasp on her arm, Narcissa halted as well, though she wanted nothing save to get through that door to Charley.

"I can hardly bear to tell you..." Rachel Hughes spoke hesitantly. "They thought it best to amputate his left arm. My husband can tell you more; I will send at once to let him know you are here."

Narcissa could not take in the meaning of her words. She could see in her mind's eye Charley's hands, strong to rein in a runaway horse, gentle to bandage a hurt kitten. He was going to be a doctor; he would need both his hands.

Ann McMillan has written three Civil War era mysteries featuring white American Narcissa Powers, black American Judah Daniel, and British correspondent Brit Wallace.

Dead March - first in series
Angel Trumpet
Civil Blood
Chickahominy Fever
Castle Thunder (Kindle edition)