Answer
Yes, it is correct that the work is proportional to $Q^2$.
Work Step by Step
The work done while bringing in another bit of charge depends not only on the amount of charge you are bringing in, but how much is already present. The result is a $Q^2$ dependence.
This answer and its mathematical proof rightfully belong in the next chapter. The work needed to complete the process is the electric potential energy of the stored charge. Equation 24.9 shows that the energy is proportional to $Q^2$.
