In the book Neri kills Moe Greene's death happens months before the killing of Tattaglia and Barzini. Shouldn't this have been a signal to Barzini that the Corleones aren't as weak as they seemed?

I always thought so.

In the novel, Moe doesn't mention Barzini at all. All he says is that he has "friends who can back me up." He even threatens to go to New York and talk to the Don, making him an offer.

The guy had plenty of chutzpah, I'll give him that .

Well, killing Greene was an obvious move by the Corleones that Barzini probably anticipated. Besides, the Corleones were co-owners, so getting rid of a partner isn't such a drastic move in the underworld. In any case, it wouldn't change Barzini's perception of Michael as a mediocre successor and his plans to usurp the Corleones pre-eminent position.

I'm curious about this...

Am I correct in assuming that the characters of Hyman Roth and Moe Greene were loosely based on Meyer Lansky and Bugsy Siegel,respectively?

To a large extent, yes.

Great! Thanks, Lilo.

Of all the characters in the films and novels, Roth and Green (and Fontaine) are the most like their real-life counterparts. The novel says Green was shot to death at the home of his Hollywood mistress. Bugsy Siegel was shot to death at the home of his Hollywood mistress, Virginia Hill.