If you want to cross reference to another DocBook document, then you cannot use xref and link, because they require the target id to be in the same document. In DocBook, a document could be a collection of books contained in a set element. If you can put all your content in one set, and you are willing to work with such a large document, then you can use the regular cross reference elements within the set.

The more flexible solution for linking to other documents is to use olink (outside link). See Chapter 23, Olinking between documents for a complete description of how to use olinks.