Buy Coggan and Allen's book. You'l want it if you buy a power meter anyway,and it goes over the different power meters.

IMHO: 1)SRM if you can afford it;

2) Ergomo or Powertap SL or 2.4.
The ergomo has the advantage of using mulitple wheels. PT the advantage of multiple bikes.

3) powertap pro.

Below that (Polar or Ibike) I wouldn't bother.