Employee motivation is often an area of interest for HR and LR practitioners. While an interesting debate can be had about the political economy of employee motivation, most practitioners are more interested in practical advice.
Most HR texts do a poor job of handling motivation. One of our learning designers passed this video of Dan Pink discussing employee motivation. It summarizes the literature of motivation set out in his book Drive. It is also exceptionally entertaining to watch.
Another good book on motivation is Driven (by Paul Lawrence and Nitin Nohria) examines motivation from the perspective of evolutionary psychology.
-- Bob Barnetson