Rands in Repose: One Thing. A good description of some of the psychology behind the Deming principle of minimizing work in progress.
You cross the threshold of inbox usefulness when you begin to mistake the act of managing the importance rather than acting on the importance. Speaking as a human who has crossed this uselessness threshold multiple times, I am prepared to declare that I am 100% done with productivity products. There is a better, simpler, and more productive way.