I recommend the following book - listed from least technical to more technical:
XML - Step by Step, Second Edition, Young. Good intro to XML and XML-related technologies like CSS and XSLT. Includes basic try-it-yourself exercises that only involve using the IE browser and editing some fairly simple text documents - no programming required. Early chapters and introductory paragraphs of later chapters give very good description of XML technologies - including plenty of business-level material as well as IT/tech-level material.
XML and Java, Second Edition, Developing Web Applications, Maruyama et al. This book includes very technical material with extensive examples using Java but the introductory paragraphs of several chapters are excellent - even for the less technical. The chapter on mapping XML to relational databases is the best available on this topic. This book is really targeted for architects and lead engineers - sharp technically, could do it themselves, but primarily responsible for the technical work of other people. It is a hard read for the technical manager - lots of code examples and tech-talk - but the tech manager willing to scan chapters will find plenty of "gems".
Building XML Web Services for the Microsoft .Net Platform, Scott Short. The early chapters and introductory paragraphs of later chapters are very good on a variety of XML topics - and not overly .Net biased. Later chapters get more technical and (naturally, some material is very .Net specific. This book is similar to the Maruyama book in that the target audience isn't general management but this book is much less technical than Maruyama - it will regularly get into architect/developer material but is much easier read for tech interested managers.
XML Schema by van der Vlist and XSLT by Tidwell. Briefly cover the basics of XML (well-formedness, validity, etc.) and jump right into using these specific technologies. These are both developer books but, in these topics, the best available. Recommended to system engineers or very technical managers if they have a need for more info about these specific XML technologies - schema and XSLT. In that case, early material is really good about "how" (ignore the rest of the book) but still a little weak on "why".
XML - How to Program, Deitel et al. One of the most popular XML books for programmers and developers. This is a very "nuts and bolts" book. Good detail, broad coverage of technical topics - perhaps a little too broad in venturing off to material that really isn't about XML. A very popular developer book but entirely wrong for architects/managers.
I wrote a review of the Maruyama book - email me if you want a copy (email@example.com).
I also really like Goldfarb's XML Handbook of (course) and and currently reading Daum's System Architecture with XML.
Increasingly, I keep this information on the web at http://del.icio.us/brianlawler/XML+References