  Bright gamma-ray flares observed from sources far beyond our Galaxy are best
explained if enormous amounts of energy are liberated by black holes. The
highest-energy particles in nature--the ultra-high energy cosmic rays--cannot
be confined by the Milky Way's magnetic field, and must originate from sources
outside our Galaxy. Here we summarize the themes of our book, "High Energy
Radiation from Black Holes: Gamma Rays, Cosmic Rays, and Neutrinos", just
published by Princeton University Press. In this book, we develop a
mathematical framework that can be used to help establish the nature of
gamma-ray sources, to evaluate evidence for cosmic-ray acceleration in blazars,
GRBs and microquasars, to decide whether black holes accelerate the ultra-high
energy cosmic rays, and to determine whether the Blandford-Znajek mechanism for
energy extraction from rotating black holes can explain the differences between
gamma-ray blazars and radio-quiet AGNs.
