The title

I love the title, but I confess I had no idea what it meant. Cain's explanation (via Wikipedia) is this:

According to Cain, screenwriter Vincent Lawrence spoke of the anxiety he felt when waiting for the postman to bring him news on a submitted manuscript. According to Cain, Lawrence noted that he would know when the postman had finally arrived because the postman always rang twice, and Cain then lit upon that phrase as a title for his novel. Upon discussing it further, the two men agreed such a phrase was metaphorically suited to Frank's situation at the end of the novel.

With the "postman" being God, or Fate, the "delivery" meant for Frank was his own death as just retribution for murdering Nick. Frank had missed the first "ring" when he initially got away with that killing. However, the postman rang again, and this time the ring was heard, when Frank was wrongly convicted of having murdered Cora, and then sentenced to die for the crime. The theme of an inescapable fate is further underscored in the novel by The Greek's escape from death in the lovers' first murder attempt, only to be done in by their second one.

In his biography of Cain, author Roy Hoopes also recounted the conversation between Cain and Lawrence that gave birth to the novel's title. Hoopes' account of their conversation is similar to Cain's, but offered extended detail regarding Lawrence's comments. Specifically, in Hoopes's telling, Lawrence did not say simply that the postman always rang twice, but rather said that at times, he was so anxious awaiting the postman's delivery that he'd go into his backyard intentionally trying to avoid hearing the postman's ring. However, Lawrence continued, this tactic inevitably failed because if the postman's first ring was not noticed, he would always ring again, and, even from the backyard, that second ring would inevitably be heard.

In the 1946 film adaptation of the novel, Frank explicitly explains the title in the terms offered by Hoopes' biography of Cain.

A suitable title? Anything to add?