Reddit mentions of The Power Meter Handbook: A User's Guide for Cyclists and Triathletes

Not an endorsement for TP but this offers some help to get started with PM training and it is free so read up on it a bit: http://home.trainingpeaks.com/blog/article/how-to-start-training-with-power-ebook

Also I believe Joe Friel has an excellent book and he tends to lay things out in an easy to read and understand way (again not an endorsement): https://www.amazon.com/Power-Meter-Handbook-Cyclists-Triathletes/dp/1934030953

The best way to think of it is this, you can judge your fitness based on a few things: Speed, Heart rate, or watts.
Speed is the simpliest(I'm getting faster! I'm not getting faster.) but it can be effected by a lot; wind, terrain, drafting, aerodynamics, ect.
Heart rate is the next when used with speed(I'm getting faster and my heart rate isn't exploding!) but like speed it can be effected by alot too. Are you sick today, not rested, to much caffeine, along with all the things effecting speed. There is also a lag between effort and heart rate(If you do a 30sec or less effort your heartrate will only see a change near the end or after.
A power meter or watts is the most effective. In short if you are producing more watts, you will be going faster, longer or both. And the things that effect speed does not effect watts. And unlike Heartrate there is little to no lag since it is measuring the effort you are doing.

I'd suggest reading Joel Friel has some great books about using power meters: Training and Racing with a power meter or Powermeter Handbook

All the answers you need are here:

http://www.amazon.com/The-Mountain-Bikers-Training-Bible/dp/1884737714

As /u/milliken says, having a structured training plan is definitely beneficial. I'm just now starting to develop my plan as I get back into racing after a long layoff, and I've found this book indispensable.

FWIW, I also recommend:

• http://www.amazon.com/Lactate-Threshold-Training-Peter-Janssen/dp/0736037551
  
• http://www.amazon.com/The-Cyclists-Training-Bible-Friel/dp/1934030201/ref=pd_sim_b_5?ie=UTF8&refRID=1BHJ1VR61WPGYDBP8TAC
  
• http://www.amazon.com/Training-Racing-Power-Meter-Hunter/dp/1934030554/ref=pd_bxgy_b_text_y
  
• http://www.amazon.com/The-Power-Meter-Handbook-Triathletes/dp/1934030953/ref=pd_bxgy_b_text_z
  
• http://www.amazon.com/Precision-Heart-Training-Edmund-Burke/dp/0880117702/ref=sr_1_1?s=books&ie=UTF8&qid=1427498205&sr=1-1
  
• http://www.amazon.com/Serious-Training-Endurance-Athletes-2nd/dp/0873226445/ref=sr_1_5?s=books&ie=UTF8&qid=1427498205&sr=1-5