Good XeTeX Books

XeTeX is in opposite to ConTeXt just another compiler engine (like PDFTeX) and not a separate typesetting system. So it can't do any harm to read LaTeX documentation. A kind of XeTeX documentation is given with the manuals of some specific packages.

• fontspec
• mathspec
• polyglossia
• xltxtra
• xunicode

I'm sure you will find much more useful information on the XeTeX Project Homepage.

1. Many books about LaTeX in general are mentioned here.
2. Documentation about XeTeX can be found here.

If you're interested in XeLaTex and read French, you may find this book useful: Maïeul Rouquette, (Xe)LaTeX appliqué aux sciences humaines, 2012. The code is open source.