Definitely buy the book recommended above. A powermeter is a novelty if you do not know how to use the data it provides. And if you do not use the data, all it does it add weight to your bike.

If you ride more than one road bike I would recommend a PowerTap so you can easily train with power on both bikes. If you ride one road bike then either a crank based power meter or a PowerTap(assuming you will just use that one rear wheel) will work well.

PowerTap is updating their hub lineup and you may want to wait until their new hub is released.