It has been hard to attempt to sustain a global view while writing this book. Whatever the failings, we feel confident that the condensing of the subject into one useful set of references will provide something that has not been available before. Probably more a book to dip into than one to read end-to-end, we hope also that the diversity of materials will prove both stimulating and informative, allowing readers to draw their own conclusions and make their own connections. For the enquiring new student or the seasoned professional, the text is scattered with leads, many of which should cut down on wearisome surf time. Hope the book help students and research scholars benefit a lot.