The Great Treatise as a Whole
Although Robert Thurman is disposed to ebullience, I wholeheartedly agree with his assessment that Tsong Khapa's Great Treatise is "one of the greatest religious or secular works in the library of our human heritage." It is a masterpiece of clarity, beauty, and utility, presenting the great scholar-yogi's overview of the stages of practice based on sutra and the Indian and Tibetan commentaries, from its initial stages to the its most profound contemplations on the final nature of reality.
Deriving from the Indian tradition of Prajnaparamita commentarial literature, the Great Treatise offers a comprehensive overview of Buddhist thought, organized into a graded series of contemplations and practices and presented in a progression ranging from the fundamental practices that can be understood and carried out by anyone, and proceeding in an ascending hierarchy to more demanding practices for beings of "higher capacity."
The practices are generally presented in the sequence in which any yogi would undertake them, beginning with refuge, moving on through contemplations of the inevitability of death, and on to the development of compassion and so on, though the principle organizational principle is how each practice is regarded with respect to the necessary capacity to undertake it, as well as the degree of resolution it affords into the nature of reality. For example, the practices of concentrating the mind in meditation are the second to last topic in the series, but it is one of the first practices any yogi would need to undertake, and serves as a foundation for most of the rest. Without meditative stability, a yogi cannot focus sufficiently to undertake the various contemplations laid out in this treatise.
Generally speaking, the Great Treatise, like the rest of the Prajnaparamita literature, is treated by Gelukpa monks more like a encyclopedic compendium of practices and positions than an actual series of practices to be programmatically followed - at least according to Georges Dreyfus's observations. Nonetheless, the Great Treatise certainly can be taken as a core guide to practice, and I daresay that approach is particularly useful for lay people.
It is also particularly relevant for people who wish to undertake practice based on sutra and Indo-Tibetan commentaries, rather than taking the tantic path. Following the example of the Bengali Kadampa reformer Jowo Atisha, Tsong Khapa scrupulously distinguishes between the practices based on sutra and the practices based on tantra, emphasizing that they should be kept separate. Despite ultimately being concordant, the way they talk about the same concepts is sufficiently different that blending them is a cause of deep confusion, in his view.
I have read a great deal of Tsong Khapa's writing, and this work in particular stands out to me for its beguiling clarity and profundity. I can get into hashing out the differences in the object of negation as posited by the Svatantrika-Sautrantika-Madhyamikas versus the Prasangika-Madhyikas as much as the next guy, but this work is of obvious and great immediate existential relevance to anyone. It is superbly translated by a team of terrific scholars, most of whom have produced other writings I have greatly enjoyed. It is a masterpiece, and a cornerstone of the Tibetan tradition.
Volume One
This volume presents an extensive introduction to the topic of Buddhist practice as understood by Tsong Khapa, using Jowo Atisha's brief "Lamp on the Path to Enlightenment" as a guide. He discusses the fundamentals of practice such as how to conduct meditation and why it is worthwhile, and presents topics relevant to people of "lesser" and "middling" capacity, including the contemplation of the inevitability of death, an analysis of karma and reincarnation, and a close look at the kinds of suffering engaged by beings of different types.
I'm not crazy about the description of these practices as "lesser" or "middling", as it is easy to get the wrong impression. I would prefer to say that they are "fundamental," necessary as a starting point for anyone on the Buddhist path, and underlying all subsequent practices. Renunciation may not seem as exciting a topic as profound emptiness, but without a deep realization of what it entails, no higher insight will be possible.