A few weeks after Ray Tracing Gems became available for free on Kindle, the book is now free on Google Play. From the book description:

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This book is a must-have for anyone serious about rendering in real time. With the announcement of new ray tracing APIs and hardware to support them, developers can easily create real-time applications with ray tracing as a core component. As ray tracing on the GPU becomes faster, it will play a more central role in real-time rendering. Ray Tracing Gems provides key building blocks for developers of games, architectural applications, visualizations, and more. Experts in rendering share their knowledge by explaining everything from nitty-gritty techniques that will improve any ray tracer to mastery of the new capabilities of current and future hardware.

What you'll learn:

• The latest ray tracing techniques for developing real-time applications in multiple domains
• Guidance, advice, and best practices for rendering applications with Microsoft DirectX Raytracing (DXR)
• How to implement high-performance graphics for interactive visualizations, games, simulations, and more