I have the book on my Kindle, but am yet to read it. I'm curious as to how my weight would affect my triathlon performance, especially as that is a sport full of compromises. Huge cyclists thighs won't help you much on the run, neither will carrying lots of chest and arm muscles, but what is the fastest spread of muscles overall across the total race.
After completing a few 70.3's and not doing horrendously, I completed my first full IM a couple of weeks ago, and keen to improve on what I considered to be a relatively mediocre time. I'm putting the training in at around 18 hours a week, but 5'10" and 10st I'm on the light side. Carrying a bit more bulk might be the key to a bit more speed.
You may wish to consider investing in a set of fancy scales. I have the Tanita BC1000 which are a bit flash as they are ANT+ and you interrogate them with a compatible Garmin watch. You do get some fantastic trends to upload to Garmin Connect though as seen below. While the body fat may not be 100% accurate, it should be repeatable, so despite perhaps being out by a couple of percent, it will give you consistent comparative info.
It will also be able to tell if you put on weight, what sort of weight you are putting on - water/muscle/fat or in a strange scenario, perhaps even bone
As The Swedish Chef has already said, I know when I'm in form when people tell me that I look ill, or my particular favourite, cadaverous