Walpola Rahula's short book What the Buddha Taught is among the best introductions to the essential doctrines of early Buddhism that I have read. His introductory chapter admirably lays out the basic "attitude of mind" that characterizes Buddhism. This is followed by four chapters, each on one of the Four Noble Truths: the fact of suffering or dissatisfaction (dukkha), cause of dukkha, the cessation of dukkha, and the actions by which one brings about the cessation of dukkha. Rahula then devotes a two chapters on topics that are critical for the appreciation of Buddhist enlightenment. The first of these chapters deals with what is possibly the most important concept unique to Buddhism among the world's venerable religions: the doctrine of the non-existence of the self (anatta). The second chapter is devoted to the cultivation of the mind through meditation.
In the chapter on anatta, Rahula appears to be responding to a work that had been recently published by Christmas Humphreys. Humphreys translates a passage in the Dhammapada as asserting "The Self is the Lord of the Self." From this, Humphreys goes on to reject the Theravada interpretation of the doctrine of anatta, claiming that their are two selves: one that is the composition of psycho-physical attributes and another that is "a reservoir of character brought over from life to life" which while not immortal, endures long enough to "control the lower self" and in a long process of self-purification, become liberated from the suffering world. Against this, Rahula argues that the translation that Humphreys accepts is faulty. Instead, Atta hi attano natho should be translated "One is one's own refuge" or "One is one's own support." The passage does not suggest a bifurcation of the self. On this score and on others, Rahula's interpretation of Buddhism is in accord with the doctrines of Early Buddhism, i.e., the Buddhism that pre-existed the Mahayana reformation.
In the final chapter, Rahula discusses how this ancient practice can be made relevant to the modern world. Though written more than 50 years ago, his account is still quite relevant. A significant portion of it refers to the Sigala Sutta and is devoted to recognizing the importance of loving and respecting ("worshiping" the people in one's life in place of the six directions that were traditionally worshiped in the ancient Indian practice, but Rahula also discusses what is a lay Buddhist's proper relationship to economics and politics.
Finally, Rahula includes translations of five important suttas and excerpts from five more. These are generally the suttas that he has referred to in the course of his book.