can someone just clarify something about SOAP for me...

In the simple calculator examle I've downloaded, the server side code is written as a class module, yet i'm asked to deploy a WSDL document on the web server.

This WSDL document seems to be rather like an XML document, but where does it come from? (It's not created when I compile the Class, it was just included with the example code.)

Do I have to have a WSDL document, or can I just use the resulting DLL?

Is there a good book anybody can recommend?