Once you get over the rather trivial nature of the split in history that inspires this novel (Lincoln was sick, so we couldn't have the War Between the States), it quickly becomes an engrossing and exciting adventure tale involving an industrially stunted Confederate States of America, a politically stunted United States of America, a dynamic British Empire where spying (the alternative to war) is still conducted by gentlemen, and a fledgling Nazi Germany. Actually, I probably should not characterize the split in history as "trivial" (that's reserved for Avram Davidson's alternate history tale where events went sideways because Brigham Young stubbed his toe), but as "slim." The Republican Party split from the Whigs solely on the issue of slavery, and the abolition of slavery was the only plank in the Republican platform, so Lincoln's sudden illness would seem to me a bit insufficient to stop the political juggernaut that led to war; still, momentous events have turned before on just such small actions. This novel rarely delves into politics, but we can surmise much from the comments and the timeline. The split between the states would also have been a split between the parties, with the Republicans (maybe Whigs, if the failure to end slavery doomed the new party)running the USA and the Democrats grasping the reins of the CSA. While the USA did not entirely avoid conflicts (Washington state was lost to Canada), it did seem to avoid the Spanish-American War because there were no Democrats beating war-drums; similarly, the CSA fared poorly against Cuba and lost Florida. The USA did not enter the Great European War, and no one cared what the CSA did there because it was so ineffectual, more of a troublemaker, I assume, than a combatant. At one point, a character visiting the USA War Department remarks about all the empty office space -- "I suppose that's what comes from not fighting so many wars."
The USA at the time of the novel is semi-isolationist and rather politically/diplomatically naive from the viewpoint of the Brits, who had to fight the Central Powers and save the day pretty much on their own; the CSA is a sort of corn-pone Sparta with a hereditary dictator and lacking the industrialized economy that would make slavery ineffective, a country shunned by everyone except the Nazis...well, when your best friends are the Nazis you have more problems than you can even count.
It is this developing "axis" between Richmond and Berlin that moves the story, propelling action after action...in some of those actions we see echoes of our own history (can't always escape destiny, I guess), the motivating force behind the characters, who include a freed slave, a Confederate chess-playing soldier, and a British secret agent who was supposed to come in from the cold...but didn't. Not a lot of Finch-Molloy's (the spy) background is revealed, but a later book, Red Wheels Turning, tells of his recruitment and an exploit during the Great War.
Since alternate history novels are in a sense historical fiction we come across quite a few familiar names, especially with regards to the Germans. All the big-hats are there, including the little chap wearing the Charlie Chaplin mustache, as well as Dr Hugo Eckner, of Zeppelin fame, but the airship bound for Cordelle (not Lakehurst) is named for Bismark, not another famous German. As with other AH novels, you can approach this story from any number of avenues -- there we go but for the grace of God, if only we had ourselves been so prescient about the Nazis, what does this story say about us, or what else has gone differently to create such a world with such people; but I think the most rewarding way to approach this novel is as a rattling good adventure novel filled with such characters that would make our own world a better place to live...if only they were real.