Authoring in XML is a two-step process: first you put information into XML, then you get it back out. The advantage of XML over other authoring processes is that you have a lot more flexibility in how you get it back out.
Turning that flexibility from thoretical possibility into practical reality requires tools with enough power to do interesting work and a thorough understanding of how to customize those tools to do the work that's actually interesting to you.
In the DocBook world, the DocBook XSL Stylesheets are a very commonly chosen tool. I won't assert that they're the most commonly chosen, but neither would I be surprised to find that they were.
The best way to gain a thorough understanding of the stylesheets it to buy Bob Stayton's excellent DocBook XSL: The Complete Guide and read it.
The guide provides a well-paced introduction to stylesheet customizationIn fact, it's likely a useful book for any XSL practitioner, whether you use DocBook or not. leading the reader from the simple, straightforward changes that can be achieved simply by setting parameters, to quite complex topics like title page layout and configuring internationalized generated text. The guide covers both HTML and XSL FO (print) customizations.
Bob is a long-standing contributor to the stylesheets themselves and a generous and diligent participant on the DocBook and DocBook-apps mailing lists. This has given him keen insight into the areas where inexperienced authors and stylesheet writers are most likely to stumble. The guide pays special attention to those areas, offering detailed explanations of modularity, profiling, cross-document linking, verbatim listings, and table handling, for example. There's even a discussion of Website, a fairly radical customization of DocBook designed for producing web sites instead of documents.
As someone who cares a lot about DocBook, I think it would difficult to overestimate the value of DocBook XSL: The Complete Guide. I'm confident it will make the stylesheets accessible to a whole new community of users.