The three volume omnibus is called "The Domination" by S. M. Stirling - the link is to the extensive Wikipedia article. And here's the blurb from Amazon - £5.99 on your Kindle app.
"In this omnibus of Marching Through Georgia, Under the Yoke and The Stone Dogs, S.M. Stirling traces the rise of the Domination of the Draka and its long struggle with the United States and the American-led Alliance for Democracy. In this alternate history, the Americans who rejected the Revolution did not scatter to the four winds and Canada; a slight change in the course of events brought them to the new Crown Colony of Drakia instead, starting with the Cape of Good Hope and soon encompassing the whole of the right subcontinent of Southern Africa. Burning resentment and limitless ambition spurred the expansion of the people who called themselves the Draka.The story is told from the point of view of an elite Draka plantation family. It's unsettling but there's something rather admirable about their Spartan-inspired culture and special-forces style 'can-do' attitude; you do find yourself rooting for them as they take on the Nazis. Good people trapped in an evil system.
There they built the Domination, an empire of inconceivable wealth and savagery, founded on conquest and slavery. It grew until it spanned the African continent, and then in the Great War of the early twentieth century, to include much of Asia as well. In the Eurasian War, Germany and the Soviet Union exhausted each other and the Draka stepped in, leaving the Domination triumphant from the English Channel to the China Sea. Only the United States and its allies stood between the Draka and their dream of an enslaved humanity.
By the 1990s, the Draka commanded the stuff of life itself, mastering biotechnology until they could create new species at will... and transform themselves into the Master Race of their savage dreams. The Alliance for Democracy traveled another path, into the mysteries of the physical universe and the technology based on such knowledge. The final confrontation would settle which was more powerful... or leave the earth a lifeless rock where nothing human remained but bones."
In Stirling's Wikipedia article, it is observed:
'Stirling frequently uses the Draka and other villains as point-of-view characters, leading to complaints that he has some sympathy with them. He is known to be dismayed by this analysis of his work. He describes the Draka series as dystopias based on "suppos[ing that] everything had turned out as badly as possible, these last few centuries". Stirling responded to these accusations in his novel Conquistador, which contained the quotation (variously attributed to Larry Niven or Robert A. Heinlein) "There is a technical term for someone who confuses the opinions of a character in a book with those of the author. That term is idiot."'Anyway, book one, Marching Through Georgia where we accompany the liberal-minded Draka Eric von Shrakenberg in the Caucasus, is a great read and I'm enjoying it.
Update (Sept 19th 2015): the second volume, Under the Yoke, is incomparably darker.