This is the most up to date article about xStats. It describes the current methodology and the descriptive and predictive qualities of the stats. It also delves into a quick and dirty method for adjusting the stats using a histogram of their exit velocities.
This article was featured on the Hardball Times on March 9th, 2017, and it is even John Sickels Approved. I consider it a must read for anyone even tangentially interested in xStats or Statcast in general.