What is ergativity?
This book gives a small definition of ergativity as " a grammatical pattern in which the subject of an intransitive clause is treated in the the same way as the object of a transitive clause, and differently from transitive subject."

Did this definition help you? It did if you are interested in languages and linguistics. No doubt!
If you are interested in languages, then this book should be obligatory reading. It can help us understanding languages in a different way.

Prof. R.M.W. Dixon has done research on ergativity in several countries since at least the 1970's.

What is most interesting in this book is the discussion of how languages might get or lose ergativity and different kinds of ergative split.

We as speakers of non-ergative (i.e. acusative) languages like English might think that our way of expressing is either the only one that is logical. This book shows that there are other ways of organizing words in clauses.

Some languages show ergativity in only certain clauses. This applies for example to the Caucasian languages in general (from the three groups: South, Northeast and Nothwest). This book gives only the example of Tsova-Tush (or Bats). What lacks here in the book is showing that they have ergative features in mostly one aspect and tense, aorist. Other clauses might even have the subject in the dative case.
Split ergativity even applies to most of the Indo-Iranian languages from Hindi-Urdu to Kurdish.
However, as the languages that have ergative systems belong to different languages groups the author gives examples mainly of those he was involved in research like for instance in Australia. ( )