I would write my own summary but I think Rab1953 did a wonderful job at that already. I will only add, having read a lot of Jewish history and familiar with the Yiddish language, I think Chabon does a masterful job at capturing that dark humoured, pessimistic European Jewish sentiment. His crusty, unlikeable but funny characters are perfect. He incorporates so much of the culture and worldview that defined European Jews in their unstable existence in the centuries leading up to the Holocaust. Even though the story is zany, the attitudes of the Jews he describes are believable. This is one heck of an alternative history. Not to mention Chabon is an incredible writer and provides amazing prose.
I wonder if people unfamiliar with the history, culture and language he is describing can really appreciate how 'spot on' Chabon is. In any event, great read.