The Path of Freedom (Vimuttimagga) / Upatissa Thera -- Kandy, Sri Lanka: Buddhist Publication Society, 1961.

The Vimuttimagga is among the more important early Buddhist texts.  Written in the 1st or 2nd century and attributed to the Theravadan monk Upatisssa, it is a foundational manual for meditation.  As with so many early Buddhist works, much of it is rather opaque and requires a strong background in Buddhism to be profitably read.  Still, a number of points easily rise to the surface.  The first six chapters (roughly 20% of the whole) cover a range of subjects preliminary to meditating, e.g., virtue, the setting for effective meditation, and the conditions for effective concentration.  

The remaining six chapters take up the actual practice of meditation, beginning with an account of the 38 subjects of meditation.  These include colors (red, blue-green, white, and yellow), earth, air, water, and fire, various "putrescences" (e.g., bloatedness, discoloration, dismembered bodies), and death, along with more uplifting subjects (e.g., loving-kindness, compassion, the Buddha, the dharma, and the community of monks).  Upatissa explains how and why one might meditate on these subjects and who might benefit from them most.  

He provides a very readable account of the stages of meditation.  Each stage provides an increasingly refined and enlightened state of consciousness.  Stage one involves the sustained application of thought.  Ordinarily, our thought process jumps from idea to idea, subject to subject, seemingly of its own accord.  It is what Buddhist frequently call the "monkey mind," jumping from branch to branch. In the first stage of mediation, we calm the mind and gain control over our attention.  This is best achieved in a quite and secluded place.  As we achieve mastery of our thoughts, we enter the second stage of meditation that is characterized by joy.  This second stage is more refined than the first stage in which we must make an effort to sustain our attention to subject of meditation.  To further refine our meditation and enter the third stage, we must transcend joy.  Doing so, we experience bliss which in turn must be transcended to reach equanimity in stage four.  

Upon perfecting stage four we transcend the realm of form and enter the realm of formlessness.  Here, we pass through additional states of consciousness: the sphere of infinite space, the sphere of infinite consciousness, the sphere of nothingness, and finally the sphere of neither perception nor non-perception.  Each of these states are more refined than their coarser predecessors. 

Throughout the work, Upatissa weaves in accounts of numerous fundamental ideas in the Buddhist tradition such as the "ill" (unsatisfactoriness) of life, the impermanence of all things, and the non-existence of the self.  He provides a fine account of two of the most important Buddhist doctrines: dependent arising and the Four Noble Truths. 

Few works in the Buddhist canon provide such an accessible, practical, and sweeping account of both Buddhist concepts and the crucial Buddhist practice of meditation.  Still, it is by no means an introductory work.  A reader should only attempt it when they have a robust understanding of Buddhism.