Any true fan of alternate history, especially with an ancient flavor, should read S. M. Sterling's Nantucket Trilogy (Island in the Sea of Time, Against the Tide of Years, and On the Oceans of Eternity). They're all linked here: http://www.amazon.com/Island-Sea-Tim...ews/0451456750

And when you're through with that masterpiece, you should go on to his related trilogy, set in modern times but where most fundamental processes of contemporary technology no longer work (Dies the Fire, The Protector's War, and A Meeting at Corvallis]). Link here: http://www.amazon.com/Dies-Fire-Roc-...e=UTF8&s=books

Both trilogies highly recommended, just don't read them out of sequence.