You might check out TEI Publisher , a free, open source tool that implements the TEI Processing Model  for indicating how you want to transform your TEI into different media, including EPUB:
I am looking for a tool converting TEI
files to EPUB files.
Does anyone have any experience in the
choice of a tool among the ones available?
Note: I tried http://www.tei-c.org/ege-webclient/
but I got an error message during the converting process.
Thanks for your help.
Bibliothèque de Genève