Where / how do these people get the specs? If they can get the specs then so should I. Can you please suggest a link / a book or a something where I could get developer specs for the mp3 format?

I had once worked on a utility for automatically updating tags from musicindia website.... I found this opensource java based project to be a very good starting point.. it has classes for MP3 files IDE tags etc.

http://mediachest.sourceforge.net/

you may also look at KDE multimedia sources if you are comfortable with C++