Walter Mosley's Advice About Dialogue

Every time characters in your novel speak, they should be:  (1) telling us something about themselves; (2) conveying information that may well advance the story line and/or plot; (3) adding to the music or mood of the scene, story, or novel; (4) giving us a scene from a different POV (especially if the character who is speaking is not connected directly to the narrative voice); and/or (5) giving the novel a pedestrian feel.

(From This Year You Write Your Novel, by Walter Mosley, pp. 88-89.)

Where some writing books are long-winded, making writers work to find nuggets of information, Mosley's is short and filled with practical advice.  Check it out.