This information is from a co-worker of mine named Patrick Dawson, reprinted here with permission…
Mike Kay’s “XSLT Programmer’s Reference” (ISBN: 1861005067), 2nd ed., by Wrox Press is often regarded as the definitive guide on XSLT.  I used the first edition religiously on a couple of projects.  However, this is probably not an ideal book for beginners.  It’s more of a reference than a tutorial.  I dislike the way it’s written (it can be inpenetrable in places, as many Wrox books are), but the accuracy and completeness of its content can’t be beaten.  Kay is also very active in XSLT mailing groups and is the developer of one of the fastest Java XSLT processors around.  See, and be sure to use the second edition instead of the first. 
Khun Yee Fung’s “XSLT:  Working with XML and HTML” (ISBN: 0-201-71103-6) is a nicely organized introduction to XSLT.  In 2001, I recommended this book to clients and Navigator consultants who were new to XSLT and wanted to quickly learn the 20% of XSLT that you’ll use 80% of the time.  This book delivers that within 50 or so pages.  Its big drawback, however, is that it is now somewhat out of date.
O’Reilly Press, which generally produces fine Java and XML books, has a couple of recent XSLT offerings that weren’t around when I last did XSLT project work.  I can’t vouch for them, but they might be worth a look.  Try the following:
Oddly enough, Ken Holman’s training materials for Crane Softwrights continue to be widely recommended as a great instructional source on XSLT.  In mags like the “XML Journal”, readers have voted his stuff as being great.  I’ve read his free excerpts and liked them, but I haven’t purchased any of his online publications.