XQuery is being developed by the W3C XML Query working group as a standard query language for XML. It is a fully compositional, strongly typed functional language to flexibly select, recombine, and restructure XML documents and fragments. This paper introduces IPSI-XQ, a comprehensive implementation of XQuery "by the book", which closely follows XQuery's formal semantics, realizing all steps of the abstract XQuery processing model in an open and modular way to arrive at a fully conformant, type safe implementation. It describes how the formal specification can be deployed for type analysis and query optimization, and discusses the pros and cons of the approach.