I suggest having a minimum price per hour that you know you can live on and a medium and high price too. Use the high price on the risky jobs. Ask for the safe medium price on the easy jobs. All too often you get into jobs where you basically only get your min price even when you bid your medium price because of details not seen. You are far better off under promising and over delivering so keep your prices fair so you can do some thing extra as you find the need when the job progresses.
Bidding is as much an art as a science. No two jobs are the same and you never stop learning on how to estimate better. Keep the risk reward in mind when setting price. More risk = more reward
While I highly recommend those books the DVD on the subject would be worth wild too as they are funny and motivational. They are not however the end all.