Robert J. Alexander wrote this magisterial tome in 1991: "International Trotskyism 1929–1985: A Documented Analysis of the Movement".
At 1,141 pages and out of print, it is a labour which will surely never be repeated. However, the PDF exists on the Internet and you can download it here.
The history of the International Marxist Group, the IMG is covered in pages 492-496. The quote from my previous post, on the occasion of Martin McGuinness's death, is on page 493.
There is a detailed and somewhat critical review of the book here (1993).