I would get the book before spending more. If you aren't the type to download the info from your rides, and spend some time analyzing the data then save your money.
You could also read through the google group:
http://groups.google.com/group/wattage
A lot of the questions there are very technical, but still it gives you some ideas on training.
The nice thing about Powertap is that if you buy, then decide you don't want to be bothered there is a good market for used models and you can get a lot of your money back.