This is a fairly complicated question, because it depends on your investing horizons and your ability to handle risk.
In the book, he basically states that if you have a long-term investment plan and can tolerance a certain degree of risk, it is better to invest the lump sum all at once for several reasons.
1. Markets are efficient and it is impossible to predict consistently where the market will be tomorrow.
2. It takes the emotion out of the timing to invest.
So, invest your lump sum as soon as possible and be done with it!