Where is 'createdb'?

This is probably a question for the authors of a book I have been reading but it may be faster to get an answer here.

I was trying to follow along in a book ‘Seven Databases in Seven Weeks’ and chapter 2 deals with PostgreSQL. One of the first things it does is issue a command ‘createdb book’. The text before this command says, “Once you have Postgres installed, create a schema called book using the following command: $ createdb book’ But when I tried to issue this command (at a Linux command prompt) I get ‘createdb command not found’. Are the authors out of date? What is the current command?

Kevin