Practics-oriented book on implementation of a basic ray tracing system:
Kevin Suffern, "Ray Tracing from the Ground Up", AK Peters, 2007.
Thorough description of a complete physically-based rendering system:
Matt Pharr, Wenzel Jakob, and Greg Humphreys, "Physically Based Rendering, Second Edition: From Theory To Implementation", Morgan Kaufman, 2017.
Detailed survey on advanced light transport algorithms and theories:
Philip Dutre et al., "Advanced Global Illumination", 2nd edition, AK Peters, 2006.
Theory-oriented, comprehensive overview of image synthesis:
Andrew Glassner, "Principles of Digital Image Synthesis", Morgan Kaufmann, 1995.