There were, necessarily, many things that didn't make it into the final manuscript, in spite of the book's long, long length -- at the time of writing, judging only by spine width, it's one of the longer volumes in the series. Most of these things were likely only of interest to obsessives; but then, I suppose if you're coming to this after having purchased the book (thank you), you may qualify.
Some links and resources follow. There are also a few pieces on the blog.
Chapter-by-Chapter Additional Materials:
One Week // One Band
A couple months after the book was first published, I was lucky enough to spend a week over at One Week // One Band; and was able to write there about some of the things that I had left on the cutting room floor, including:
An extended riff on Portishead's aesthetic treatment of samples, particularly from soundtracks
Their dramatic sense of dynamics, particular as they have re-worked one song ("Over") over many years
How Third confirms their deliberately confounding sensibility: Portishead want you to feel uncomfortable
The band's visual branding; and some aspects of their influence
Interviews, Reviews, and Excerpts
Upon the book's publication, Continuum ran two excerpts on the 33 1/3 series blog.
I was interviewed by Anupa Mistry for the Toronto Standard: Why Portishead Still Matters.
Later, the 33 1/3 ran a Portishead week, covering the band’s visuals, their influences from film soundtracks, the use of their music in film, and their short film To Kill a Dead Man.
The Process
And for the true inside baseball, I wrote about the process of pitching to write a 33 1/3 book.
And where the project started: