I've found Liz Castro's book XML for the World Wide Web: Visual QuickStart Guide (Berkeley, CA: Peachpit Press, 2000) to be a concise yet thorough treatment of the subject. Liz writes good books, and I've found this one particularly useful.
XML in a Nutshell, 2nd Edition (Sebastopol, CA: O'Reilly, 2001) by W. Scott Means and Elliotte Rusty Harold also offers a concise treatment of the subject material and even gets into some of the more esoteric areas of the language.
Steve Holzner's Inside XML (Indianapolis, IN: New Riders, 2000) is also a good read. It's comprehensive and covers many key XML subjects in great detail.
Erik T. Ray's Learning XML (Sebastopol, CA: O'Reilly, 2001) is another good introductory text. It contains ...