PARIS — In France, they say they’re puzzled by the humor. In Germany, they say it will be difficult to market. Martin Amis’s latest novel, “The Zone of Interest,” a satire set in a concentration camp during the Second World War, is having trouble gaining traction in Europe, where his longtime French and German publishers have rejected it.The novel was published by Jonathan Cape in Britain in August to strong reviews and will be released in the United States by Knopf on Sept. 30. By turns a love story and a meditation on Nazi horrors written with self-consciously grotesque humor, “The Zone of Interest” takes place in the fictional Kat Zet I, the same fictional branch of Auschwitz where Mr. Amis set his 1991 novel, “Time’s Arrow.” The new book is certainly not the first work of fiction to treat the Holocaust with dark humor. But in Europe, where there has been particular sensitivity recently to a rise in anti-Semitic incidents, publishers this time seem squeamish.