An author I like that I haven't seen mentioned here: SM Stirling..

[Fumesucker]

Stirling tends to do alternate history, my first exposure to his work was "Drakon" which is the last of his Domination of the Draka series. The Draka are some truly nasty villains who nevertheless are rather sympathetically portrayed by Stirling as a good deal more than one dimensional, they have a code of honor which they vigorously uphold. The Draka universe is probably rather unlikely by soc.history.what-if standards but is a truly chilling one despite that.

http://en.wikipedia.org/wiki/The_Domination

http://en.wikipedia.org/wiki/S._M._Stirling

I just started a book by Stirling that I haven't heard of before, "The Sky People", it pays homage to ER Burroughs in that Venus is an inhabitable world with indeed something very close to humans indigenous to it as well as everything from carnosaurs to pteranodons to dire wolf analogs to something very like a sabertooth tiger.

Knowing Stirling, this will all be explained in a hard SFish manner as the book progresses. The western nations and the eastern block have both started colonizing Venus at the time of the book and evidence so far points to something having caused Venus to change from what it like in our universe to something very earthlike in about the last 200 million years.

If you like military SF and are not squeamish about graphical depictions of violence and somewhat less graphic sex you might like to give Stirling a try.

Stirling collaborated with Anne McCaffrey on "The City Who Fought", another book with some truly nasty villains which also uses McCaffrey's "shellpersons" from "The Ship Who Sang", he has also collaborated with David Drake and others..