Sorry I've been away... jury duty In fact, it may keep me away for the remainder of the week.Since I feel that the Roman Question has been adequately dealt with above, I shall endeavour to answer the secondary question, where my expertise truly lies: Books on Alternate Histories.
(A concept of which I am a great fan)
All of these I have read, and recommend:
"How Few Remain" -- Harry Turtledove's story of the SECOND War between the states. ("Guns of the South") was not really a prequel to this, and is more of a fantasy/time-travel story. I don't generally include these.
"The Great War: American Front"
"The Great War: Walk in Hell"
"The Great War: Breakthroughs" -- the first three novels in Turtledove's 4-novel Alternate WWI, based on the events of "How Few Remain." US-Germany vs. CSA-UK-France, with a Black communist rebellion in the CSA thrown in for good measure.
"Moon of Ice" -- Brad Lineweaver,s novel of nazi victory in WWII.
"Hitler Victorious" -- A collection of Nazi victory tales, some more fantasy than others. The "Captain America" one is the most fun, I think.
"What If?" described above.
"What If" -- A drier, more historical look by military historians as to how different decisions and outcomes during WWII might have changed the course of the war... or not.
"What Might Have Been, Vol. I-IV, -- edited by Gregory Benford and Bob Greenberg, a voluminous collection of alternaty stories from dozen of authors.
"Alternate Presidents"
"Alternate Outlaws"
"Alternate Warriors"
"Alternate Kennedys"
"Alternate Tyrants" -- ed. Mike Resnik. More collections of stories by various authors.
Number of the Beast" & "Job: A Comedy of Justice" -- Robert Heinlein. Although not truly 'Alternate Universe' stories in their entirety, each visits several interesting 'Alternate' worlds, and "Job" is worth reading as satire and for its religious discussion alone.
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"Ed Gruberman, you fail to grasp Ty Kwan Leap. Approach me, that you might see." -- The Master