A lot of tbook code is documented by DTX files. The command make doc transforms them into PDF files. tbookdtd.pdf is the best starting point; it contains succinct documentation of the tbook DTD, although the description of elements you can also find in section Elements reference. tbookxsl.pdf contains the documented source code of the XSLT stylesheets. But that file and even more the other PDFs that used to be DTX files have very poor documentation/source ratio.
(If you comment out the \OnlyDescription in tbookdtd.dtx,
you get a list with most available entities and their LaTeX
realisation.)