Matt PharrHappy to report that the full contents of the 4th edition of Physically Based Rendering are now freely available online: https://pbr-book.org/4ed/contents
Per Vognsen@mattpharr Not sure if an error but the name of the "A Brief History of Physically Based; Rendering" section is either a typo or a very heterodox typographic convention.
@pervognsen thanks! It's a mis-parsing of a TeX macro by the HTML generator, so it's some of each in a way... Will fix!