XSLFO lists: the yahoogrouops xsl-fo list and the w3c xsl-fo list.
Dave Pawson's XSL-FO book (buy or read online)
Ken Holman's XSL-FO course and book
Eliot Kimber's Using XSL Formatting Objects for Production-Quality Document Printing presents the state of xsl-fo renderers
An interesting looking article on xsl-fo that I haven't had a chance to read closely yet: What Is XSL-FO and When Should I Use It?