SM Stirling is the author of a number of books in alternate Universes.
See here for S. M. Stirling's biographical information.
He has been working in the science fiction and fantasy fields for quite a while now. However, many of the earlier books and series have been co-authored, with many of these being with David Drake in the General series. He has also written a shared novel in Anne McCaffrey's Brain/Brawn universe as well as a follow up novel of his own in the same universe
As an individual, his greatest impact on the scene has been in the alternate history sub-genre, most particularly the Drakaverse and the Island in the Sea of Time universe. Island in the Sea of Time in particular provoked a lot of discussion on the soc.history.what-if newsgroup forum, closely followed by his Draka books.
The Drakaverse explores the possibilities of what might have happened if the British had lost the American Revolutionary War even worse than they did in our timeline. In these novels, the British lost all of their holdings on the American continent and those who would stay loyal to the King were resettled in the Cape Territories, taken more thoroughly from the Dutch. The society that evolved from these displaced persons makes the grim realities of Apartheid look like a tea party. Apart from the unlikeliness of this society rising to such prominence as swiftly as they do, the thing that really got people going was the way in which Mr Stirling makes the Draka appear so sympathetic.
The books are:
The Domination, which consists of the three novels Marching Through Georgia, Under the Yoke, and The Stone Dogs, which tell the rise of the Draka from their humble beginnings to worldwide domination.
Drakon, which is set more than four hundred years after the Final War. Beaten on Earth, the Alliance fight back from Alpha Centauri but mostly in a primitive New York in an Alternate Universe that has to deal with infiltration by a lone Draka and an agent from the Alliance!
In contrast, the Island in the Sea of Time books take up the discussion of what would happen if a relatively limited number of people were flung back into the deep past. This has been done with individuals all the way back to Mark Twain's A Connecticut in King Arthur's Court. However Mr Stirling decided that a single person was unlikely to have enough of an effect for what he wanted, so he sent the island of Nantucket careering back in history. About three thousand years back, in fact!
The trilogy looks at the efforts of Nantucket to survive in this strange new world.
The books consist of the following:
Island in the Sea of Time Opens the saga
Against the Tide of Time Natives and dissidents Nantuketers cause the Island and it's allies problems
On the Ocean of Eternity With all their enemies in disarray, the Nantucketers are able to dream of a starry future
Meanwhile, what's been happening 'back home':
Dies the Fire Mankind has to accommodate itself to the loss of Technology
The Protector's War Now things are stabilising, more wide-ranging plans can be made
A Meeting at Corvallis. The final confrontation between the two ways of life in the New Earth
The on-line discussions of this series mainly revolved around whether the Nantucketers would actually have the skill set represented in the books and then whether they would be so dominant.
Not series currently, but with plenty of opportunity:
The Peshawar Lancers What would happen if a cometary impact wiped out North Hemisphere civilisations?
Conquistador When John Rolfe explored the world of Ham Radio, he got more than he bargained for...