You can validate your tbook files with nsgmls, an SGML validator that can be called by Emacs for you. It will show error messages, because it bases on an XML declaration file that is erroneous. Try to find this file (on my system it's called xml.dcl, but xml.decl is also possible) and change the line
128 32 UNUSED
128 32 128
You may also simply ignore those error messages.
But this modification will also help Jade which stands behind the docbook2?? translators. By the way, Jade may complain about namespace attributes, but this too is false alert.