In addition to the above you should take a look at the book "Basic Techniques of Photography, Book 2" by John P. Schaefer. This contains a really easy way to do densitometry without a densitometer. It is the clearest and most reproducible of all the systems I've tried.(posted 9081 days ago)Book 1, incidentally, is the best single volume about photography that I've seen.
Both books have the sub-title "The Ansel Adams Guide" and are available as large format paperbacks. Amazon.com has them and so does my local mega-bookstore.