XML is a huge topic to learn and understand, and these books offer both the beginner and the advanced programmer something to use. While there are many books on specific XML languages and specifications, these books are some of the best all-around XML information manuals. There are books for people who learn by example and for the more studious minded.
1. 'Beginning XML' by David Hunter et al
This is a very comprehensive book for the beginner. In fact, it might be a bit too comprehensive, as it's size might be intimidating. But this book is well worth it. If you're brand new to the concept of markup languages, this is a good book to pick up, but it isn't light reading. I recommend treating each chapter as a separate book. Read each section to understand the ins and outs of XML.
2. 'Learning XML' by Erik T. Ray
If you're a Web developer who understands HTML and wants to learn more about XML, this is a great book to pick up. It covers all the basics of XML including markup, links, DOM, transformations, and more. It is also a great book for developers who want a refresher on the basics of XML and markup.3. 'XML by Example' by Sean McGrath
This is one of the first XML books I picked up. I like the fact that it really is a book of examples, rather than assuming that you want a long explanation of what XML is. The first 30 or so pages of the book covers XML and what it is and then you jump right into the examples. There is so much in this book, and once you know the basics of XML, this is a must have.4. 'Applied XML Solutions' by Benoît Marchal
One of the most common questions I get about XML is "but what can it DO?" This book answers that question. Often if you have some examples of how others have used XML you can put that to better use in your own work. You will learn how to use XML to store your data, create electronic forms, export and import data to multiple formats, and much more.5. 'XML in a Nutshell' by Elliotte Rusty Harold and W. Scott Means
This is not a book for learning XML. But if you already use XML, you will find it an invaluable reference. There is a quick overview of the basics of XML and DTDs, and then it provides overviews and references on all sorts of XML related topics: namespaces, XSLT, XLinks, CSS, and more. Finally there is a comprehensive reference section covering XML 1.0, XPath, XSLT, DOM, SAX, and character sets.6. 'XML by Example' by Benoît Marchal
This book makes XML fairly easy to understand and use. It takes you through XML in a step-by-step approach that allows you to understand each concept before you move on to the next. One of my favorite sections is the part on "common errors and how to fix them". This can be a lifesaver when you're frustrated. It's fairly short, but it covers what you need to know.7. 'Mastering XML' by Chuck White, Liam Quinn and Linda Burman
This is a very hefty tome. But with all the topics it covers, this is not surprising. It starts out with the basics of what XML is, and how it can be used to structure your data. Then it moves into some of the more popular XML standards, XML development tools, and even case studies of how people are using XML in the real world. This is really several books in one.8. 'Professional XML' by Didier Martin et al
This is not a book for the faint of heart. While it does cover the basics of XML and all you would expect from an XML book, it is very dense. The book is huge, and the pages are thin, so there is a lot of information stored here. If you're looking for a book to bridge the gap from HTML to XHTML, this is not it - but if you want a programmer's view of XML, then pick it up!9. 'XML Bible' by Elliotte Rusty Harold
Another very large book, the XML Bible is a very comprehensive book. I find it to be a bit fluffy at times, but it does contain all of the necessary information you need to learn to use XML. It also includes some interesting uses for XML, like creating local language versions of your Web sites.