The subtitle is a bit misleading since, as the book points out, about 2/3 of German Jews did leave Nazi Germany before 1940. Having said that, the book did teach me something I didn't know: over half of those Jews left during the last couple of years before WW 2, rather than leaving right after Hitler took power.
The book seeks to answer the question: Why did Jews dawdle? Because at first, it was unclear how bad Nazi oppression would be; in the mid-1930s, it seemed quite possible that Jews could still make a living in Nazi Germany. But by 1938, the situation was more desperate, and most Jews tried to leave.
If anything, the author fails to explain not why Jews stayed, but why so many were able to leave in the last years before the Holocaust. In 1938-39, most large countries were unwilling to accept a significant number of Jewish refugees. Yet over 100,000 German Jews managed to somehow leave Germany during those years. A better book would have explained how they managed to do it, and what separated the successful emigrants from those who tried to leave but couldn't.