[lxml] xpath example

I am in search for a good xpath example. Let say I have, this xml file <?xml version="1.0"?> <catalog> <book id="bk101"> <genre><s>Computer</s></genre> <price><f>44.95</f></price> <publish_date><d>2000-10-01</d></publish_date> <description><s>An in-depth look at creating applications with XML.</s></description> </book> </catalog> How can I extract only the price of a book which has a genre of "computers"? How can I extract, price, description of book id"bk101" ?