The TEI DTD has found wide acceptance in the academic world for marking up texts and documents, and you are encouraged to use it as an alternative to the Gutenberg DTD's. Although the initial 'learning curve' is quite steep, it becomes much easier as one goes along.
Most book DTD's are designed to enforce a certain structure on the author. The TEI DTD was designed for marking up any existing document in any language in order to facilitate their analysis. For this reason it has a very loose structure, and uses generic section dividers to seperate out sections of documents.
Furthermore it was originally written when computing and storage resources were at much more of a premium than they are today, so the tag names tend to be a little terse, and not very intuitive. However in defense of TEI the original TEI DTD allowed for renaming of the elements to suit the taste (and language) of the author. The teixlite DTD does not allow this.
The full TEI DTD is a huge DTD, and the official documentation runs to over 1400 pages! However it is possible to do useful work with this DTD using a much smaller sub-set, and the following pages provide a brief tutorial that is designed to give you enough to mark up your first prose or poetry book using the XML version of this DTD, the teixlite dtd.
Hopefully the following pages can also be used as an introduction for those wishing to learn the TEI DTD. There is also an excellent tutorial written by Lou Burnard and C. M. Sperberg-McQueen which can be used to complement this material. If you are serious about learning TEI you are encouraged to print out or save this material to act as a handy reference.