First some words about where to get help. The text you are reading is available in Info format with
in HTML (tbookman.html) in /usr/share/doc/tbook/, and in PDF format (tbookman.pdf) (e.g. on tbook's homepage). There are manpages available for tbtolatex, tbtohtml, tbtodocbk, tbprepare, and tbook itself.
A browsable DTD is stored in the /usr/share/doc/tbook/ directory, which may be found at a different place depending on the options you chose during installation. But it's also accessible online at http://tbookdtd.sourceforge.net/dtd/.
If all else fails, use the forums on tbook's homepage. See Web addresses.