Thanks for the summary and confirming you're looking at the second edition. I only asked about the edition since in the second edition he notes that the first edition contained a number of errors in the film speed section.
I'm far from being an expert on ISO, ANSI and the history of speed determination (including the various other methods going all the way back to H&D) so it was hard for me to evaluate Henry's text until he gets to in-camera testing etc. Light falloff and the possible effects on in-camera testing with contacted step wedges had also occurred to me when I first read Howard Bond's article on film reciprocity failure. In my own in-camera EI testing it had previously never even occurred to me to use a step tablet in the camera.
Since I'm not all that knowledgeable when it comes to ISO speed testing, I found it hard in Henry's book to make what seems (to me) like a leap from the ISO discussion right back into an in-camera test using a grey card, placing it on Zone V, closing down 4 stops etc. to determine a working EI. There seems to be a disconnect there in the text. But I guess he just wanted to go through the proper theory first and then proceed to some more "real world" EI tests.
One thing that still puzzles me. If one does that typical Zone System EI test with the outdoor grey card, is there a difference between metering the grey card and metering a white card?