Once there was a little boy who loved computers. He loved them all: electronic calculators, his grandpa's totaling cashier machine, and of course the PC. Once he got into a plant that made computers and saw the assembly line. How he wanted to play with them all! But he realized that it did not make sense. Was he going to walk from one to the other, and do the same thing on each one?
And then the boy grew up, became a software developer, and he found Hadoop. Now he had what he wanted! Clusters of computer, all his! He could start the clusters and shut them down at will.
But Hadoop was a hairy animal, hard to control, and maintaining it was a real skill. So the boy decided to write a book, "Big Book on Running Hadoop."