In the novel, Maurice, the French intelligence officer, speaks a few idiomatic French phrases. Some are in argot, or slang, the others are regular everyday speech.
This expression literally means "isn't that so" and is used at the end of a phrase or sentence. In idiomatic English, we would usually say "right." For example, in French, the expression la fille est jollie, n'est-ce pas? would translate into English as: the young lady is pretty, right?
Go to my page, What Does This Mean?, for more expressions in French. :)