It used to be the case that the best way to get AppleScript was to buy System 7.5! At that time there was also an AppleScript Developer’s Kit for $199 which included the available Addison-Wesley manuals, FaceSpan, and code for making your applications scriptable and recordable. Thank goodness times have changed.
Now AppleScript is included with every shipping Mac, and most of the Macs that you will find used or in use. To write a script you need two pieces: a script editor (you can find Apple’s own into your “AppleScript” folder) and the AppleScript component (in the Components folder in the System/Library folder).