This is one of the worst books I've read so far this year. The characters have no depth, the plot is silly (and doesn't even really go anywhere until 80% into the book), and the author doesn't even seem to care that much about the Civil War.
Sophie and Jamie are a present-day married couple. She's very ill and is waiting for a heart transplant when one day she just totally vanishes. She's transported back to 1863 Pennsylvania while Jamie is left in Oregon wondering where she is. He tries to move on, even seeing a grief counselor.
When Sophie wakes up in 1863, her heart is no longer damaged. You might think that waking up suddenly in other era with no assets would be a problem, but Jackson isn't interested in exploring this at all . . . she's found by a local wealthy family and they simply decide she'll live with them for the rest of her life because . . . well, we're never told why. The people that we encounter in 1863 have no motivations or interior life, they're just there to make sure Jackson never has to think about how Sophie will actually survive. A local military trainer, Richard, falls in love with Sophie. In fact, almost everyone falls for Sophie in one way or another and most of the book is everyone praising her, falling in love with her, or simply just wanting to make her life easier.
Sophie is supposed to be a Civil War buff, but she's remarkably incurious about being transported to the era that is supposedly her favorite. We never see her discussing the war in detail, reading a newspaper, or trying to interact with the time in any meaningful way. She volunteers at a military hospital, but only because a new friend of hers spends a lot of time there. She doesn't seem to know anything about 1860s life. She's surprised people expect her to ride sidesaddle, she's astonished when people don't treat her like a 20th century woman, and she's incapable of understanding their standards of proper dress or avoiding using modern slang that confuses her new friends. Eventually Jamie shows up in 1863, but for reasons that are never explained, he doesn't remember who he is. For some reason, he has no memory of his modern life, but he was able to seamlessly integrate into 1860s life (despite the fact that it was Sophie who liked the Civil War and not him) and even jumps right into a role as a heroic Union officer (although his unit never really goes anywhere besides Gettysburg and he's always around to take Sophie on picnics).
Eventually we find out why Sophie and Jamie are in the past and it's ridiculous . . . she was sent back because she's responsible to making sure that another group of modern people -- aligned with the Confederacy -- don't change history so the Union loses the war. How this mysterious group that controls the "time portal" chose Sophie makes no sense. We're told that it is because she knows so much about the war -- but keep in mind that this is a woman who didn't even realize the Battle of Gettysburg was approaching until she overheard some people saying that it looked like the Confederacy was planning to attack there.
You might think that Jamie and Sophie might have some meaningful conversations on how they could help their new friends (or their nation) given her historical knowledge, but no. Sophie has no interests other than riding a horse and sneaking off with Jamie. Oh, she also sings, regularly, to rapturous applause each time (at one point we're told that a group of dying soldiers are mesmerized by her singing and beg her to come back the next day). At about 80%, the author seems to realize that she forgot to have much of a plot so some bad guys show up and presumably the plot is continued in future books.
Even if you can set aside Sophie's ignorance of the era she supposedly loves, the author herself doesn't seem to know much about the era. For example, at one point, a character (not from the future) refers to "Miss America," which didn't even exist until the 1920s. Unless you totally buy into Sophie and Jamie's supposedly great love, there's just nothing here for the reader (and if I had to read "Ten Cow," his stupid nickname for her, one more time, I think I would have thrown my Kindle at the wall).