Not sure. The links from the index.html file on my computer all point to the /usr/doc directory, rather than /usr/share/doc (see attachment of screenshot with chapter one, noting the url), so once the index file was properly found on my machine, it then hyperlinked to the other html files in /usr/doc. Perhaps you're using a newer version from testing. Again, I'm using the version in the stable repositories.
The commands I ran started in the /usr/share/doc directory. You may have run the commands from another directory, such as your /home/user directory, which may have screwed up the placement of the links.
This linking of files should not be necessary. It's a bit careless to place the documentation files in the /usr/doc directory. Hopefully they'll fix this on xarchiver in version 0.5.2-i586-4vl70.txz