I'm glad I read this one. Though many of the principles were familiar (both from The Essential Drucker, which I had read previously, and from various management concepts which are taught by others today), many weren't.
The book's focus is on making the 'knowledge worker' (as opposed to a manual labourer) more effective. Whereas a manual labourer's efficiency can be easily measured, this is not so for the knowledge worker, requiring some additional effort in ensuring effectiveness.
The first thing Drucker argues is that effectiveness is something separate from intelligence and work ethic. Highly intelligent people can be ineffective, and vice versa. But effectiveness can be learned, according to Drucker.
The second thing to understand is that the knowledge worker's most important resource is time. This is the one resource that cannot be replaced; therefore, it must be organized such that this time is optimized. The book offers a number of insights into how this can be done.
I've been inspired by Drucker to try some of the steps he suggests. One of the first steps is to record how one actually spends his time, because it is usually quite a bit different from how we perceive we spend our time. I tried an app to do this, but I've found it incredibly difficult to record my time accurately; this probably speaks to how much I'm switching between tasks, which likely isn't good for my effectiveness!
I highly recommend the book to any knowledge worker interested in improving his productivity.