The XML Bible. IDG, 1999.
Harold, Elliotte Rusty.
The best book on XML around. Harold explains things in a clear, concise way. If you'd like to learn XML, this is the book for you.

XML Pocket Reference: Extensible Markup Language. O'Reilly & Associates, 1999.
Eckstein, Robert.
This is a great little book. Since it's a Pocket Reference from O'Reilly, it's a small book that you can literally carry in your pocket, but it is jam-packed with useful information. You wouldn't use this book to learn XML, but once you know it, this would be a fantastic resource.