The official information on XQuery  is comprehensive and
detailed, so much so in fact that it counts more than 170 printed pages. A more tutorial
approach can be found in the XQuery book by Wamlsley 
which gives good examples of use and tricks of the trade. Another excellent source of
examples are given in the XQuery Cookbook . It also
gives examples of use of for the
eXist database and how XQuery can be
used with programming languages and environments.