For XSL, XSLT and XPath, start with the W3C recommendations at,
This includes a section on FO (formatting objects),

Lars Marius Garshol indexes most available XSLT engines at

The latest information on tDOM is at

For SAX, see

Cocoon information starts at

Learn more about scripted documents at

I keep hyperlinks and other information and opinions pertinent to my own XSLT projects at