Atisha was an established teacher at Vikramashila, one of the great monastic universities of eastern India, when envoys arrived from the kings of western Tibet asking him to come north. The invitation had been pursued for years and at considerable cost, and the tradition tells it as a story of gold raised and a king dying in captivity so that the ransom could be spent on the invitation instead. He agreed to go for three years, arrived in 1042, and never returned to India.
What his hosts wanted was not more texts. They had a great many, translated over two waves, tantric and scholastic, contradictory in places, with no account of how any of it fitted together. What Atisha wrote for them is a short work of some sixty verses, the Lamp for the Path to Enlightenment, and its achievement is organisational. It sorts practitioners into three capacities according to what they are actually aiming at, the better rebirth, personal liberation, and the liberation of everyone, and then assigns each teaching its place in that order, so that the whole inheritance becomes a sequence rather than a heap.
Every later stages of the path text in Tibet, across all the schools and including Tsongkhapa's enormous treatise, is built on that framework. A short book written to answer a practical question from a remote patron turned out to determine the shape of Tibetan Buddhist education for the next thousand years.