XForms Essentials by Micah Dubinko, O'Reilly, 2003
XForms: XML Powered Web Forms by T.V. Raman, Addison-Wesley, 2003
XForms for HTML Authors by Stephen Pemberton
This presentation: http://www.cafeconleche.org/slides/sd2005west/xforms/
XForms 1.0 specification: http://www.w3.org/TR/xforms/
Top 10 XForms Engines: http://www.xml.com/pub/a/2005/02/09/xforms.html