A big topic.
Reading that book would be a good start. Also ‘The Lion’s Roar’ by Chogyam Trungpa is good–it goes into the yanas that come before Vajrayana too, which he considered important.
I have read the Gampopa book–very interesting.
I particularly like the part where Milarepa leans over and whispers in Gampopa’s ear, saying “There IS a Supreme Being!”