Hey
There is actually one excellent book for Supras - “Celica & Supra: The book of Toyota Sports Coupes”
"My Book" is really a long collection of PaperWork that Toyota Filed with FIA between Jan 1987 and the end of 1989. It was originally purchased from FIA by JOSE ,and later he scanned it and gave it to me to use in my Group A research. There is not much there other than many technical specs of cages ,arms ,brakes that toyota used in competitive racing back in the days.
There is also one pretty article on the subject of Supra in Group A racing ,writen by a friend of mine (Francisco Cunha) and hosted on Frank's site:
http://touringcarracing.net/Media/Toyota%20Celica.pdf