I actually got more out of working through example code than from any book on SOAP. The "Programming Web Services with Perl" book may be useful however (I didn't have that book when learning).

I would start with the SOAP::Lite documentation and Cookbook here:
http://guide.soaplite.com/
http://cookbook.soaplite.com/

Work your way through those simple examples. Getting them up and running will give you a good starting point for your own code.