For your flights, know that San Sebastian airport is actually right next to the town of Hondarribia, right at the border with France (you can actually walk into Hondarribia from the airport). It is a very small airport with limited flight options. If you can't make flights into San Sebastian work, Bilbao has a larger airport with many more options for connections. From Bilbao airport, there is a bus direct to San Sebastian (details in Rick's book).
A third option is flying into Biarritz airport in France. I don't know how easy or hard it is to get from that airport to San Sebastian.
From the city of Bayonne (as close to the Biarritz airport as Biarritz itself is), there are direct buses, and an easy train connection, although the train does require a half-block walk between the end of the French train line and the start of the Spanish train line.
If you want a nonstop flight from the US, your only options are to Paris, Barcelona or Madrid. None of these are particularly close to San Sebastian, so you'll probably want at least one overnight in the arrival city before heading there.
Summary: from the US, getting to the Basque country is definitely NOT half the fun. I just want to prepare you now, so you'll know what to expect when you start searching.