The closest book that would cover this would be Professional XML Databases (yes, a Wrox book). However, like the others, I have to ask, WHY?
Is it doable? Sure. But it will also be slow and cumberson. I would highly recommend you talk your customer out of it. If they want to use XML, they can use the new XML data type that comes with SQL Server 2005. Very fast, powerful, and still allows for "relational xml data".