BW, I think that is the original '20 rep squat' template. I would run an internet search before buying the book, I can't recall off-hand, but I think it is like the Smolov, where it is easily accessible on the internet. If I remember, it is 6 weeks, squatting either 2 or 3 times a week depending on your recovery.....assistance was minimal, little to nothing, thats the program with the pullovers right after the squats.
Two approaches guys use is one, to take the 12-rep max, do a 20 rep breathing set, then go up from there.....the other is to take the 5 rep max, knock 60lbs off, and do 20's for 6 weeks finishing with the old 5rm for a set of 20 on the final workout.
Dig around the net before you buy the book though, because I think it is floating around out there.
You're already doing progressive 20 reppers tailored to suit yourself, so again, all a book provides is a basic template, that you'd most likely have to tweak a bit anyway, and it'd most likely just end up like what you're doing now.