It is a lot of work to write your own XSL stylesheet, but it is quite easy to customize an existing one. If you use the tools suggested in Chapter 2, Tools then you will have some default XSL stylesheets already installed on your system.
But rather than edit them directly, it is usually preferable to create a customization layer. A very simple one is used by this book; for print output, URLs are enclosed in angled brackets rather than square brackets, and for XHTML output the file extension is changed from .xhtm to .html.
For the URL modification, for example, take a look at the file ulink.xsl in Appendix A, DocBook XML source. If you are using xmlto then it will generate a custom XSL stylesheet for you if you use the -m option.