This book offers an in-depth explanation of Task-Based Language Teaching (TBLT) and the methods necessary to implement it in the language classroom successfully.
This book includes the research behind and scientific arguments for TBLT, as well as practical instructions for its implementation. It is exceedingly thorough, and rather opinionated; it demands that you don your heaviest-duty thinking cap at all times.
In short, this is not a layman's primer - but non-applied-linguists (i.e. teachers) should still read it.