I had discovered this free online book some months ago but I've always forgotten to place the bookmark on the right place (my Blog, obviously... 
), so it's time to do it...
Real World XML Web Services is a great online book written by Yasser Shohoud, one of the best free guide to understand Web Services, from the base concepts to the advanced tecniques. Only two bad things: a pdf version is not available and a chapter about WS Security is missing... however, a recommended reading!