Why it is on this list
The old standard English translation of a neidan manual. Read it beside the Mo Pai level descriptions and the borrowing is obvious: the gates, the circulations, the embryo, the spirit body, in the same order.
Also on the “The classical background” shelf
The Way of the Golden Elixir: An Introduction to Taoist Alchemy
The best short scholarly introduction to neidan. Pregadio is a historian of religion rather than a practitioner, which is exactly why he is useful here: he describes what the tradition says without needing it to be true.
Awakening to Reality: The "Regulated Verses" of the Wuzhen pian
One of the central texts of internal alchemy, with commentary. The kan-li operation that Mo Pai calls level 3 is this literature's central concern.
The Taoist Body
A scholar who was also an ordained Taoist priest, writing on how the body is conceived in Taoist practice. The best correction to the assumption that qi talk is a clumsy attempt at physics.
Opening the Dragon Gate: The Making of a Modern Taoist Wizard
The biography of Wang Liping, a modern Longmen inheritor. Structurally the closest thing in print to The Magus of Java — a hidden lineage, a living master, extraordinary claims, one devoted chronicler.
Foundations of Internal Alchemy: The Taoist Practice of Neidan
A modern Chinese master's systematic summary of the neidan process, stage by stage. The clearest published statement of the three refinements that Mo Pai's levels enumerate.
Taoist Meditation: The Mao-shan Tradition of Great Purity
The Shangqing visionary tradition in scholarly detail — the interior practice that fed into internal alchemy, several centuries before anything called neidan existed.
The Secret of the Golden Flower: The Classic Chinese Book of Life
A short neidan classic on circulating the light and gestating the spirit body. Read this translation rather than the famous Wilhelm–Jung one, which worked from a defective text and a psychological reading of it.