I would buy the one about Time Management, because I find that is something you are uniquely gifted at that I can learn from especially and make use of. It's a weak area of mine. The others are not so interesting to me.
You have so much free stuff about Time Management & Productivity on the site that ironically it becomes a huge time investment just to read it all and work it out. Would be great to see a unified approach to the topic that unlocks some fundamental principles and things of that nature.
I'm just one guy though.