I am interested to learn Space Time code. Is there any understandable books/ lecture notes/ thesis for beginners?
For a beginning engineer I shamelessly recommend this book written by my former colleagues. It does not get into some of the newer aspects of the theory such as DMT-optimality and multiuser theory. If you are an engineering major interested in the applications of algebraic number theory in this area, but don't really know much about it, then Frederique Oggier, Emanuele Viterbo, Algebraic Number Theory And Code Design For Rayleigh Fading Channels is a quick read.
For DMT-stuff there is more than enough material available at David Tse's homepage