W3C : Extensible Stylesheet Language (XSL)
XSL information at W3C : this page is the starting point to all the official W3C documents about XSL.

W3Schools
W3Schools provides comprehensive introductory content covering XSL.

XSL Transformation (XSLT) 1.0
The official XSL Transformations (XSLT) specification. XSLT is a language derived from XSL for transforming XML documents into other XML (or HTML) documents. (W3C Recommendation 16 November 1999)

XML Bible Chapter 14
Covers the October 8, 1999 proposed recommendations of XSL Transformations and XPath.

XSLT and XPath Quick Reference
Quick reference card developed by Mulberry Technologies in PDF format

XSLT Reference & Implementation charts
The site gives a reference of all elements in the W3C spec, detailing which implementations support the feature. The site focusses on MSXML (the most easily available implementation for VB programmers). The reference links to examples with the results generated by several implementations (Saxon, XT, Xalan, MSXML). This allows to compare implementations side-by-side.

Saxon: TheAnatomy of an XSLT processor
In this article Michael Kay, outlines the Saxon architecture to demonstrate the inner workings of an Extensible Style Language Transformations (XSLT) processor.

DevGuru XSLT Introduction & Quick Reference
This is a comprehensive 101-page reference source for all of the elements and functions that compose the eXtensible Stylesheet Language Transformation (XSLT) language version 1.0. This Quick Reference features real working code examples which were tested using the Microsoft XML parser version 3.0.

Investigating XSLT: The XML transformation language
The XML transformation language - Article from the DeveloperToolbox Technical Magazine