NVIDIA have just released the second edition of Ray Tracing Gems. It is a book from a wide variety of industry experts (NVIDIA, Intel, Pixar, Epic Games, Remedy, Roblox and more), with 50 chapters on a wide variety of raytracing related subjects. Even better, the digital version of the book is available as a completely free PDF download right here. All of the examples from the book are released on GitHub.
You can also enter to win a hard copy limited edition of the book right here, although a NVIDIA developer account (which is free) is required to enter. If you wish to purchase your own hard copy version of the book, it is now available in hard and soft cover versions on Amazon(affiliate link).
As the name might suggest, this is the second edition of Ray Tracing Gems, the first volume (we covered it here) is also available as a free download. You can learn more about both books at http://www.realtimerendering.com/raytracinggems/.