Interested in diving into alternate history fiction? The Onion AV Club has an unusual recommendation: steer clear of both Philip Roth and Harry Turtledove, and start with a Pulitzer-nominated but seldom-discussed 1972 novel instead.
The AV Club's "Gateways To Geekery" series recommends Robert Sobel's Pulitzer Prize-nominated 1972 novel For Want Of A Nail: If Burgoyne Had Won At Saratoga as the gateway drug for alternate history virgins:
This effective blending of the fanciful and the banal is what makes For Want Of A Nail such a good place to start. The writing style, by its very dryness, achieves a wonderful balance between the counterfactual history-which goes into tremendous depth, with endless variations springing forth from the most minor historical divergences-and the fictional wonders, to the point that when a huge corporation, the invented Kramer Associates, ends up as a nuclear power near the end of the book, it seems like the most reasonable thing in the world. For those interested in the "history" part of "alternate history," the book is incredibly well-researched and meticulous in its presentation of real-world historical figures; for those who like the "alternate" part, it's fascinating for how those figures play a completely different role in this always plausible, yet entirely unpredictable, divergent path of American history.
And if that book grabs you, the AV Club suggests a few classics, like Philip K. Dick's The Man In The High Castle and Kim Stanley Robinson's The Years Of Rice And Salt, as well as the harder-to-find Norman Spinrad novel, The Iron Dream. But the Club warns newbies off Turtledove ("his body of work is intimidatingly vast, and not very good") and says Roth's The Plot Against America "works best as literature, with its historical aspects often coming across as flat or not entirely credible." No mention of other oft-raised classics, like Fatherland. Or The Yiddish Policeman's Union, for that matter. (Although both books do get mentioned in the comments.) [AV Club]