I found this book a great start. In summary (for people like me who have to test a web API in a blinding hurry), install SOAP UI. Into the 'create' drop-down, enter the WSDL address and then enter the pay-load. Test the variables and you're off the hook. Otherwise - it covers a great many areas which make it suitable for holiday reading or a long-haul flight. The only reason it's 4 star is because sometimes I'm googling things I thought I didn't understand - only to find out the book can be a little long-winded/verbose etc without saying something is actually quite simple.