Like the title says, and get it here. A few important errata for Chapter 3 were reported by a reader, Ebor Folkertsma, and so have been fixed by Tomas in this newest PDF, as well as fixing the caption for Listing 27-2. Let us know of any bugs you find so we can fix them: erich@acm.org
Source : http://www.realtimerendering.com/blog/ray-tracing-gems-book-pdf-updated-to-1-6/