From Liquid Passion:

Sick of waiting for Daisuke who's late for their meeting, Bun lets himself get picked up by a handsome stranger, who then takes a liking to him and suggests seeing each other again. Can Bun really forget his old love and move on?

Miyamoto Kano's "Walker" universe timeline:

1. Are You Enemy?
2. Not/Love
   -- Not/Love dj - The Reason Why We Live Together
   -- Not/Love dj - Dog Days
   -- Not/Love dj - Demonic Conversations
   -- Not/Love dj - Brand New Lover
3. Heat (MIYAMOTO Kano)
   -- Under the Skin
   -- Lizards, Angels, and Daemons

The ones on ( -- ) are doujinshis. They give background or "add into" to some key points in the main story line. So you're not really missing anything even if you don't read it.