I use a program called Magic File Renamer. This allows some extremely complex renames to be done, including filenames, MP3 tags etc.. The standard version is sufficient.
It costs (I think) $15, but the demo version has full functionality and allows 100 rename operations. Given that I have very rarely needed more than one rename operation to make all the changes I need to the tracks of an audio book, including filenames and MP3 tags, this goes a long way. (Typically this involves 100-200 file renames etc as one operation).
Details are at
http://www.finebytes.com/mfr