The book covers the key web service approaches, SAX vs. DOM XML parsing, various Google, Yahoo, Amazon and several other API's. It also gives an example of screen scraping a web page that was not originally intended to allow mashups. Speaking of which the author makes it a case to remind people of potential legal responsibilities when creating mashups. I also really liked the section on SPARQL, which is an RDF query language, though I am not sure if a book on mashups really needed such a detailed explanation of RDF (maybe RSS would have been enough).
Anyways like I already said in the paragraph, I think this is a good book that is really hurt by the lack of proper editing in the examples. I found most of the typo's easy to spot for an experienced developer though. But its just annoying if the code in the book is broken here and there, even if its just double quotes that are malformed.