We made Shadow Unit #1 free at Smashwords and Amazon (and would've made it free at B&N if we could figure out a way). So far, that seems to have been the right choice. For example, in September, the month before #1 went free, we sold 16 copies of #9 at Amazon. In October, we sold 26. In November, we sold 59. It's too early to generalize, but it's promising.
To test Amazon's KDP program (which I really wish didn't call for a three month exclusive on any book in the program), I entered Dogland and made it free for a day this week. Last month at Amazon, it sold 4 copies; this month, it's already sold 11.
I'll do an update on the experiments in a month or so.