Finishing in hard cover form just in time for GTC 2019, NVidia and APress have team up to author Ray Tracing Gems, a book on real-time raytraced graphics development in the popular “Gems” format. Even better, they are making digital chapters available as they are developed, free to those with a NVidia developer account (which is also free). The chapters are distributed under the Creative Commons 4.0 International 
License and are available for download here. Unfortunately Part 5 is currently missing and parts 6 and 7 are slated to be published later this week.
Here is the current table of contents from the RealTimeRendering homepage:
PART 1: RAY TRACING BASICS, editor: Chris Wyman
- 1. Ray Tracing Terminology, by Eric Haines and Peter Shirley
- 2. What is a Ray? by Peter Shirley, Ingo Wald, Tomas Akenine-Möller, and Eric Haines
- 3. Introduction to DirectX Raytracing, by Chris Wyman and Adam Marrs
- 4. A Planetarium Dome Master Camera, by John E. Stone
- 5. Computing Minima and Maxima of Subarrays, by Ingo Wald
PART 2: INTERSECTIONS AND EFFICIENCY, editor: Ingo Wald
- 6. A Fast and Robust Method for Avoiding Self-Intersection, by Carsten Wächter and Nikolaus Binder
- 7. Precision Improvements for Ray/Sphere Intersection, by Eric Haines, Johannes Günther, and Tomas Akenine-Möller
- 8. Cool Patches: A Geometric Approach to Ray/Bilinear Patch Intersections, by Alexander Reshetov
- 9. Multi-Hit Ray Tracing in DXR, by Christiaan Gribble
- 10. A Simple Load-Balancing Scheme with High Scaling Efficiency, by Dietger van Antwerpen, Daniel Seibert, and Alexander Keller
PART 3: REFLECTIONS, REFRACTIONS, AND SHADOWS, editor: Peter Shirley
- 11. Automatic Handling of Materials in Nested Volumes, by Carsten Wächter and Matthias Raab
- 12. A Microfacet-Based Shadowing Function to Solve the Bump Terminator Problem, by Alejandro Conty Estevez, Pascal Lecocq, and Clifford Stein
- 13. Ray Traced Shadows: Maintaining Real-Time Frame Rates, by Jakub Boksansky, Michael Wimmer, and Jiri Bittner
- 14. Ray-Guided Volumetric Water Caustics in Single Scattering Media with DXR, by Holger Gruen
PART 4: SAMPLING, editor: Alexander Keller
- 15. On the Importance of Sampling, by Matt Pharr
- 16. Sample Transformations Zoo, by Peter Shirley, Samuli Laine, David Hart, Matt Pharr, Petrik Clarberg, Eric Haines, Matthias Raab, and David Cline
- 17. Ignoring the Inconvenient When Tracing Rays, by Matt Pharr
- 18. Importance Sampling of Many Lights on the GPU, by Pierre Moreau and Petrik Clarberg
PART 5: DENOISING AND FILTERING, editor: Jacob Munkberg
- 19. Cinematic Rendering in UE4 with Real-Time Ray Tracing and Denoising, by Edward Liu, Ignacio Llamas, Juan Cañada, and Patrick Kelly
- 20. Texture Level of Detail Strategies for Real-Time Ray Tracing, by Tomas Akenine-Möller, Jim Nilsson, Magnus Andersson, Colin Barré-Brisebois, Robert Toth, and Tero Karras
- 21. Simple Environment Map Filtering Using Ray Cones and Ray Differentials, by Tomas Akenine-Möller and Jim Nilsson
- 22. Improving Temporal Antialiasing with Adaptive Ray Tracing, by Adam Marrs, Josef Spjut, Holger Gruen, Rahul Sathe, and Morgan McGuire
PART 6: HYBRID APPROACHES AND SYSTEMS, editor: Morgan McGuire
- 23. Interactive Light Map and Irradiance Volume Preview in Frostbite, by Diede Apers, Petter Edblom, Charles de Rousiers, and Sébastien Hillaire
- 24. Real-Time Global Illumination with Photon Mapping, by Niklas Smal and Maksim Aizenshtein
- 25. Hybrid Rendering for Real-Time Ray Tracing, by Colin Barré-Brisebois, Henrik Halén, Graham Wihlidal, Andrew Lauritzen, Jasper Bekkers, Tomasz Stachowiak, and Johan Andersson
- 26. Deferred Hybrid Path Tracing, by Thomas Schander, Clemens Musterle, and Stephan Bergmann
- 27. Interactive Ray Tracing Techniques for High-Fidelity Scientific Visualization, by John E. Stone
PART 7: GLOBAL ILLUMINATION, editor: Matt Pharr
- 28. Ray Tracing Inhomogeneous Volumes, by Matthias Raab
- 29. Efficient Particle Volume Splatting in a Ray Tracer, by Aaron Knoll, R. Keith Morley, Ingo Wald, Nick Leaf, and Peter Messmer
- 30. Caustics Using Screen Space Photon Mapping, by Hyuk Kim
- 31. Variance Reduction via Footprint Estimation in the Presence of Path Reuse, by Johannes Jendersie
- 32. Accurate Real-Time Specular Reflections with Radiance Caching, by Antti Hirvonen, Atte Seppälä, Maksim Aizenshtein, and Niklas Smal
Once compiled the electronic version of the book will remain freely downloadable, although in what formats has yet to be determined.