MY EARLIER POSTS on Harry Turtledove led to requests for other recommendations in the alternate-history vein. That's a genre I enjoy, so here are some others you might like if you enjoy Turtledove.
Kim Stanley Robinson's The Years of Rice and Salt looks at a world in which Christianity never amounted to much once the Black Death wiped out 2/3 (instead of 1/3) of Europe. A bit mystical in places, but well-written. Interestingly, fundamentalist Islamists are still a problem in the new world.
Steven Barnes' Lion's Blood (and its sequel, Zulu Heart) also takes place in a world where Islam was triumphant, this time in an America colonized by African Muslims. The battle is between Sufis and fundamentalists.
Steve Stirling (S.M. Stirling) has written a lot of good alt-history. His Island in the Sea of Time (along with its sequels) is one of my favorites. Americans wind up back in the Bronze Age, with interesting results. I also liked his Conquistador, and his The Peshawar Lancers is pretty good, though a bit too British-Empire-revivalist for my taste. His most recent book, Dies the Fire, isn't exactly alternate history, but it's good.
As I mentioned before, I liked John Birmingham's Weapons of Choice, and look forward to the sequel.
Meanwhile, though only some of his stories were of the alt-history variety, I have to put in a plug for classic space-opera writer A. Bertram Chandler, whose stuff is being reissued now in collected form. (The first installment is John Grimes: Survey Captain). When I was in Australia a few years ago, I met some people who had known Chandler, a merchant sea captain who wrote his novels at sea, and they told me that he had a lot in common with his main character, John Grimes.
UPDATE: Reader Richard McEnroe writes about Chandler:
What a great old character he was! I had the pleasure of meeting him in NY a couple of years before he passed. Very much a figure out of one of his books, in the best way.
I wish I'd met him. McEnroe also recommends Eric Flint's 1632 -- which I should have remembered. And, of course, I highly recommend Neal Stephenson's Baroque Cycle books. Here's my review of those, from the Weekly Standard's Christmas book issue.