SXPath as a Query Language
XPath is defined by the W3 Consortium as a language for addressing parts of an XML document.
SXPath, however, provides features of a query language, because SXPath allows to formulate arbitrary requests for information from XML/SXML documents and to generate arbitrary reports from it.

(X)Querying XML in Scheme
Jim Bender's implementation of XQuery constructs for SXPath/Scheme

BizQuery
XML Integration and Query engine