Ray Tracing Overview
Ray Tracing in One Weekend series
Series of online books teaching the basics of path tracing
Dartmouth CS87 Rendering Algorithms
The course nicely fills the gap between a Ray Tracing in One Weekend style toy renderer and a more general and fully-fledged renderer that PBRT describes
Physically Based Rendering: From Theory to Implementation
Comprehensive reference on physically based rendering techniques
Ray Tracing Gems Series
Like other "gems" books, this series of books contains a collection of articles focused on ray tracing techniques for serious practitioners. It focuses on subjects commonly considered too advanced for introductory texts, yet rarely addressed by research papers.