I have numerous XML books on my shelf, as well as many SGML books. But my tastes are not what I think he needed. Nor do I think they are what many clinical or management folks need either. What they want is enough to understand what the technology can do, with enough examples and pointy brackets to explain what is needed, but not so technically overwhelming as to frighten them away.
The book is divided into four parts, each part including from 2 to 6 chapters. Part I covers XML. Part II DTD's (skip that) and Namespaces (read that). Part III is XML Schema, and Part IV is XSLT and XPath. It's a handy book for me because it goes into enough detail that I can use it as a pocket reference book, but it is also good book for non-technical folk because chapters are fairly short (20-40 pages), and each chapter provides a good overview of a single topic that can be easily digested.
This book won't make you an XML expert, but it may make you look like one to your colleagues, and it is a book that at least one expert (me) still uses.