The seven books of the Abhidhamma Pitaka, the third division of the Tipitaka, offer an extraordinarily detailed analysis of the basic natural principles that govern mental and physical processes. Whereas the Sutta and Vinaya Pitakas lay out the practical aspects of the Buddhist path to Awakening, the Abhidhamma Pitaka provides a theoretical framework to explain the causal underpinnings of that very path. Although parts of the Abhidhamma were recited at the earlier Buddhist Councils, it wasn't until the Third Council (ca. 250 BCE) that it became fixed into its present form as the third and final Pitaka of the canon.
The Abhidhamma Pitaka is divided into seven books, although it is the first (Dhammasangani) and last (Patthana) that together lay out the essence of Abhidhamma philosophy. The seven books are: I. Dhammasangani ("Enumeration of Phenomena"). This book enumerates all the paramattha dhamma (ultimate realities) to be found in the world. According to one such enumeration these amount to:
52 cetasikas (mental factors), which, arising together in various combination, give rise to any one of...
...89 different possible cittas (states of consciousness)
4 primary physical elements, and 23 physical phenomena derived from them
Another odd inclusion in the Abhidhamma, this book contains questions and answers that were compiled by Moggaliputta Tissa in the 3rd century BCE, in order to help clarify points of controversy that existed between the various "Hinayana" schools of Buddhism at the time.
This book is a logical analysis of many concepts presented in the earlier books. In the words of Mrs. Rhys Davids, an eminent 20th century Pali scholar, the ten chapters of the Yamaka amount to little more than "ten valleys of dry bones."
VII. Patthana ("The Book of Relations").
This book, by far the longest single volume in the Tipitaka (over 6,000 pages in the Siamese edition), describes the 24 paccayas, or laws of conditionality, through which the dhammas interact. These laws, when applied in every possible permutation with the dhammas described in the Dhammasangani, give rise to all knowable experience.