Now that you're sold on PL/SQL, I'd like to change gears a bit and give you some practical information on what you'll need to start programming.
First off, you'll need access to an Oracle database. Since Oracle is a product designed to be shared, it isn't necessary to have your own private copy of Oracle on your own private machine. You just need an account in an Oracle installation where the administrator will let you experiment with PL/SQL. You can use your desktop machine merely as a tool through which you connect to a database on a different machine. If you don't have that, though, you might have to set up your own Oracle database.
In the simplest arrangement, you would have the Oracle server running on a machine on your desk, where you would also do all your development. There are four things you will need:
Access to a "big enough" machine running an operating system supported by Oracle
A licensed copy of Oracle's server software, available free (with some restrictions) from Oracle's web site
A text editor
A copy of this book
Since you've already got the book, and getting #2 will also get you #3, you're halfway there already.
If you want to install the Enterprise Edition of Oracle9 i on a typical Unix machine, Oracle says you need at least the following:
256 megabytes of RAM
2.5 gigabytes of disk for software and starter database
400 megabytes (or more) of swap space during installation ...