Here, we see for-each select "book" which correlates with our main tags in the XML document, ie. <book> </book>. This means the information displayed only comes out of any text enclosed within the tags. The order-by section lets us select a topic area to use as our filter and sorts the book titles in order of the authors surname, and then by the genre both in alphabetical order.
A value-of tag prints the value of every book within the selected tag, eg. <title> </title> or <author> </author>. This is the main output method. Order-by is not necessarily required. Always remember to close EVERY tag you open.