Although the Web offers abundant reference material on XSLT, printed books are valuable for those who choose to pursue XSLT more deeply. Steve Holzner's Inside XSLT collects a wealth of useful recipes while accurately communicating key XSLT concepts. Along with its other benefits, Inside XSLT closes with chapters on formatting objects. I've found a great deal of interest in the last few months on use of XSLT to prepare PDF and related presentation outputs.
Also useful are Doug Tidwell's XSLT, Michael H. Kay's XSLT Programmer's Reference and Khun Yee Fung's XSLT: Working with XML and HTML. XSLT has quite complete reference appendices and presents extension mechanisms particularly well. XSLT: Working with XML and HTML has been particularly popular with those coming from graphics or web backgrounds, who don't regard themselves as programmers but need to get XSLT projects off to good starts quickly. XSLT Programmer's Reference is on the other end of that continuum: it's rather academic in its abstractions and leisurely treatment but is detailed and comprehensive. While I have the impression that many of today's XSLT programmers began their XSLT study with the Programmer's Reference, it is a reference and not a tutorial.
A final aid to XSLT understanding is the ActiveState Tools Corporation's Komodo IDE. Komodo, available for license fees ranging from $0 to around $300, includes useful XSLT debugging facilities that I've seen in no other product.