What If? (Robert Cowley, ed., 1999) is a collection of essays postulating counterfactuals--i.e. speculations on how the world would've turned out if just one small detail was changed. What if the Greeks lost at Salamis? What if Publius Quinctilius Varus never lost those legions? What if the Union hadn't found Lee's famous "Lost Order"? Etc.
They're fun reads, though some are far more plausible than others (e.g. I'm pretty sure if Athenian democracy had been nipped in the bud, someone else would've thought of it eventually). The whole exercise is of particular interest to me now, since my WIP is an alternate history, and one that's more a lengthy counterfactual speculation with action and character development than a fantastical alternate history like Naomi Novik's Temeraire series or Orson Scott Card's Alvin Maker books.