I think that's why I loved "The Way of the Strong" and "Underworld" so much; they are love stories, but they are least as much mistaken-betrayal-of-friendship stories. Of course gangsters have a lot of opportunity for betrayal...

I can't promise gangsters, but have you tried D.K.Broster's The Flight of the Heron and The Gleam in the North -- and, of course, Mr Rowl? ;-p

She wrote so convincingly about friendship that most of her readers assumed she was male :-D