> the tags as they are in the file

Is not actually relevant and is not an information the average XML processor even receives. If the file uses a default namespace (xmlns), then the elements are namespaced, and anything processing the XML has to either properly handle namespaces or explicitly ignore namespaces.

> A lot of XML is ad-hoc without a namespace defined anywhere

If the element is not namespaced xpath does not require a prefix, you just write

    //bookstore/book/title