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.
As far as I remember, Racing Weight won't actually tell you what your optimum weight is (although there is some discussion about typical/ideal body types for different endurance sports).
I haven't got a copy here, but I'm sure that there's a section where he says that the only way to know your optimum racing weight is through experience - it tends to be that leaner is better, but too lean or too light can also decrease performance.