Possible Clues:

• One sharing a spine with me?
• Pen pal?
• Wearing my old University hat loony grabs my partner?
• Book collaborator
• Preston or Child, for the Agent Pendergast novels
• Write together
• One's not self-sufficient creating work that's bound to be finished
• One of two writers
• One of the bookmakers only wrote down part of it
• Joint writer