justify his time is his personal viewpoint, and he can say with some authority how many books are too many to justify his writing a new one. Justify one's time means that nobody would find that no author of programming books would find their time usage justified considering the number of books already written.
Justify my time is arrogant and keeping, IMO, with the arrogance he often displayed, while justify one's time is a judgment that is dependent on market forces and the opinions of others willing to write programming books.
