Physically Based Rendering: From Theory to Implementation is online, with plans for annual updates supported by https://www.patreon.com/pbrbook/
Matt Pharr, Wenzel Jakob, and Greg Humphreys are creating Physically Based Rendering, online | Patreon

Become a patron of Matt Pharr, Wenzel Jakob, and Greg Humphreys today: Read posts by Matt Pharr, Wenzel Jakob, and Greg Humphreys and get access to exclusive content and experiences on the world’s largest membership platform for artists and creators.

And here's a blog post explaining the why and how behind it: https://pharr.org/matt/blog/2018/10/15/pbr-online.html
Physically Based Rendering, online

Physically Based Rendering is now available in an online edition--free for everyone to read.