How are you assembling the various chapters and sections of your book
together? If you are using XInclude or standard XML external entities to
assemble your chapters into a single book and then building that book in a
single transformation process, all of those pieces can be considered part of
a single 'document', and <xref/> or <link/> are the preferred link
mechanisms. If you are building each chapter in it's own transformation
process, what you propose with <ulink/> will probably be an acceptable
approach.