Get the Carson Graves book: "The Zone System for 35mm Photographers." There should be second-hand copies on Amazon or Alibris or Abe Books. I found this a very readable book which covers the subject in a very straightforward way.

Ansel Adam's book is in reality very straightforward as well; it's just that he goes into details that slows down your assimilation of what he is saying and requires a second reading. Well it did in my case