This is the classic book written by the management consultant Richard Koch in 1997. The book builds on the principle outlined by the Italian Vilfredo Pareto as early as 1897. In short, the 80/20 principle deal with the fact that a minority of causes, inputs or investments usually lead to the majority of the results, outputs or rewards. For example, 80% of what you achieve at your job comes from 20% of the time spent. The imbalance can be 65/35, 70/30, 75/25, 80/20, 95/5 or 99/1 or any numbers in between.
A “Brief” of the book can be read here.