How can I understand the phenomenon of mood better?
Are you interested in mood, emotion, or affect? Or all three?

Mood is considered a "diffuse" affective state, whereas emotions are "object specific."  If you're interested in this distinction, look up Mathias Siemer's work.  Gerald Clore also does work on mood.

If you're interested in emotions, then this is a bit harder to tell you who to look up.  There's a vast body of literature on many different things relating to emotions.  If you're interested in pure theory, look up Paul Ekman, Klaus Scherer, and Lisa Feldman Barrett.  If you're interested in emotion regulation, look up James Gross.  If you're interested in positive emotions, look up Barbara Frederickson.  The list goes on!  If you want a not particularly comprehensive overview, you can look at the Handbook of Emotions. 

If you're interested in affect (arousal and valence), look up James Russell. 

Hope this helps.