I think Ansel Adam's own description in his book "The Negative" is the best description. Buy the book and work through the chapter on the zone system doing all of the excercises.(posted 8888 days ago)I think Pickers book is pretty poor, if I recall corectly it doesn't even give you zone 1 densities to aim for, (Fred wanted you to send him your negs to read so he could make a buck)
That same book has a lot of crap about the alleged superiority of cold light heads.
Remember, Adam's was a great photographer and teacher and a great one at that, Picker was simply a businessman out to make a buck from Ansel Adam's wannabes
Mark