Intuitive

• Charge-parity violation by Symmetry Magazine
• The Mystery of CP Violation by Gabriele Sciolla

Concrete

• A nice introduction is The Theory of CP-Violation -- in as Much of a Nutshell as Will Fit on 8 Pages by Patricia Ball
• For a nice PhD thesis on the topic, see CP and other Symmetries of Symmetries by Andreas Trautner
• Great textbooks on CP violation is "CP Violation" by Branco, Lavoura, and Silva and "Discrete Symmetries and CP Violation: From Experiment to Theory" by Marco Sozzi
• Other famous books on CP violation are "CP Violation" by Jarlskog, and "CP Violation" by Bigi and Sanda

CP Violation in the Neutrino Sector

• The CP phase in the neutrino sector has not been measured yet. The newest current results can be found in https://arxiv.org/abs/1805.05917.

Abstract

• An extensive discussion of how to consistently define a CP symmetry in gauge theories can be found in Automorphisms in Gauge Theories and the Definition of CP and P by W. Grimus and M. N. Rebelo
• The Pin Groups in Physics by Berg et. al.

Why is it interesting?

A CP transformation transforms a particle into the corresponding antiparticle. A intact CP symmetry therefore means that particles and antiparticles are indistinguishable. However, we know from experiments that CP is a broken symmetry since, for example, there are more particles than antiparticles in the visible universe.