Fit To Pitch is about 10 years old. While there are things in it that are still valid, there are also things that have been superceded by newer things. So, from this standpoint, I would recommend Fastball Fitness which was released this year.
Fastball Fitness has an excellent section that describes how the major parts of the body contribute to velocity. The next section presents a number of training protocols from various organizations including the NPA. Finally, there is a section that discusses arm speed development using weighted implements.