XSLT, or XSL, is a subject that I'm no expert in but I've come across it from time to time and generally have had a hard time to really grasp the how and the why of it. In most cases I can program, or at least tweak, just about anything with very little introduction. Fixing and tweaking the XSLT stylesheets that I've come upon has been a tougher experience where I've felt myself reduced to guesswork and magic. That's not a feeling I like, so I decided to do some background studying.
A Programmer's Reference is perhaps not the first choice as an introduction to a subject, but in this case it was hard to find just where to start, and I felt that I was experienced enough to go for some core literature from the start, which also would have the benefit of being useful in a real situation as reference literature.
Since I'm a newcomer to XSLT this review will have to be both about the book as such, and also about the subject matter. Let's start with the book.
Michael Kay is certainly an authority, being the editor of the XSLT 2.0 Working Group. The book is also authoritative and extremely carefully written with an extraordinary focus on details. I did find a few typos, errors and editorials mistakes but taking the amount of text into account it's still a very, very good piece of work.
This book is not written to be read cover to cover, which I did, but it's still not a bad way to get a thorough introduction to XLST. Be prepared for quite a few hours though, I spent about 20 hours reading this book. It's entitled XSLT 2.0, and was written before XSLT was actually approved as an official recommendation which it was on 23 January 2007. I've not checked, but there are sure to be some minor differences between the final recommendation and the drafts upon which the book was written. In consequence being such a recent standard, there are very few XLST 2.0 compliant implementations in existance, so XSLT 1.0 is still very much in use. The book is careful to keep track of differences and changes, and should work well for XLST 1.0 use as well.
It's very heavy reading indeed, but if you only want to get one book about XSLT 2.0 this is very probably the one to get.
The real question though that I must raise after reading this and getting a good feel for XSLT is: Do you want to get any book about XLST at all?
XSLT is about XML transformation, or actually transformation in general. It doesn't really have to be from or to XML, it can be from plain text to HTML or any number of other combinations depending on the requirements and capabilities of parsers and processors available. This is obviously extremely useful - to be able to massage data from and to different forms, and frequently used in one-off applications and in various integration projects. The target use of XSLT is also to fit in along CSS 2.0 as a way to perform formatting for presentation that is not possible with CSS - that's why it's called XML Stylesheet Transformations.
So XSLT certainly address an important area. However, sadly, I must conclude that it's not a very good tool in my opinion. Even if supplemented with good development environments with color coded and syntax checking editors, it's still simply not very human eye-friendly. Too many angle brackets and colons one might say. Syntax does matter! The real problem though is that it's a functional programming language, not a procedural language, and this simply does not lend itself to performing complex tasks in the real world.
Functional languages focus on defining the program in terms of functions that are state-less and without variables. Everything is defined as functions without side-effects, that is to say, each call to a function with the same parameters will always return the same result. Iteration is replaced with recursion - even when iteration is the natural way to address a problem, because an iterator must be updated for each round in a loop, and you can't do that. This means that while anything can be programmed in a functional language, it must frequently be done in ways that are not well known to the majority of developers.
There's a reason why functional languages like Lisp, ML and Scheme have not become commerically successful, although loved by the academic community for decades. Basically I think it's a question about maintenance and complexity. In the real world of commercial programming, the systems must be maintained for decades by perhaps 1000's of different developers over the years. This has always been an uphill task, but no functional language with the possible exception of Erlang has succeeded in combining expressive power, with robustness, documentability and maintainability.
XLST is certainly expressive, but I categorize it as being of the class of write-once and write-only languages. Integrated with XPath 2.0 it's possible to write programs that are so smart, that even the author will have trouble understanding them the next day.
There's nothing wrong with the basic concept of defining a standard way of transforming documents between different representations, and making it possible to choose between doing the processing in the browser or on the server. It's neat and it's cool. However, doing anything but non-trivial transformations is a maintenance nightmare. That the functional programming model is very little known among main-stream developers does not make it any better.
Somehow, it feels like XSLT is 90% geared towards the internal needs of the W3C - it's used extensively to format and publish the specifications for the various specifications published by the W3C. But, this actually means as far as I can judge, that the specifiations are written in raw XML using plain text editors and doing the markup manually - something that won't exactly work for any other organization.
So, unfortunately, in the end I feel that XSLT 2.0 is a technology that's elegant, but will never be used on a wider scale. However, if you do have the situation of having many, many documents in some kind of structured format (not necessarily XML surprisingly enough) and want to transform them to XML or XML-like format like HTML, then XSLT may well be just what you need. Be prepared for a very high entrance cost though, and rest assured that as author of the stylesheets you'll have a very high level of job-security.
There are also serious performance issues with XSLT, due to the functional style of programming, compilers and optimizers have a hard time generating decent code for the underlying procedural architecture of our computers. In theory, functional programming could come into it's own performance wise as multi-core architectures become more common because it does make it easier to realize parallell computation, but today other problems overshadow, and in most cases I'm fairly sure that performance will in many cases be unacceptable with XSLT.
So, to summarize: If you want to learn and use XSLT 1.0 or 2.0, this book is probably the one to get, but you should not assume that XSLT is a silver bullet for XML transformation, there are many caveats.