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Yes you are right Mark, it does get updated as changes in the law dictates. So the latest edition is always better(contains more and some refinements) and "more authoritative". I have a copy of the 10th edition in my library and I must say it is a "must have". Thanks @law-wall for the recommendations.

That is sure right!. Thank you too @kryptocoin

Very right, though it does changes, most of the core remains much similar, but the newer editions often carries definition of many more new terms-which ain't new in the real sense- and gives it a more directional definition, and such things as expanded bibliographical coverage. The latest edition often also carries more sources and legal citations. I have almost 7 different editions here with me, but I believe there is a easy android app always updated on the Android app ecosystem. I think that is great.

So, Yes its worth it @markwhittam. Thank you.