TIS, you might want to read "Yes I Can", Sammy's autobiography. It's one of the best bios I've ever read and he discusses his friendship with Sinatra at length.

It was written in '67 or so, so it's not totally up-to-date but it is well worth your time.

BTW - When the Rat Pack was appearing in Vegas in the early '60s there were still segregation rules in effect. Davis wasn't allowed to stay at the same hotel as the other Rat Packers. Frank and Dean put an end to that.