The Theravāda Abhidhamma contains the rigorous abstract sytemisation of the teaching given by the Buddha as found in the Pāli Canon. It has two main objects: one, to analyse all phenomena into their irreducable parts; and two, to show how all of these things are related.
The early books of the Abhidhamma follow the analytic method, reducing things to their constituents, and showing primarily how they measure up ethically.
This is done principally by setting up a Matrix at the opening of the first book of the Abhidhamma, the Dhammasangaṇī, which provides a tabulation in triplets and pairs against which the various factors listed in the first two books of the Abhidhamma are measured.
A good example of how this is put in effect can be found in the third section of the Analysis of the Satipaṭṭhāna which is made in the Vibhanga.
The last book of the Abhidhamma, the Paṭṭhāna, is a huge book, which is so long it has never been written out and printed in full. The great German scholar Ven. Nyanaponika estimated that it would need around 26,000 pages, but even then would still contain many repetition (peyyāla) passages.
The book is concerned with how factors are related to each other, and begins with a simple enumeration of the twenty-four conditions that are recognised as prevailing amongst phenomena, followed by an explanation of these conditions, and it is a translation of these sections which is published on Ancient Buddhist Texts now.
The text gives a good understanding of the inter-relationships between phenomena, which is much more complicated than usually understood, and it is especially useful for understanding the deeper aspects of Conditional Origination (Paṭiccasamuppāda), and the functioning of conditionality (Idapaccayatā) in general.
The immediate reason for making the translation is that it forms a section of the Chanting Book used at the Pa Auk group of monasteries, for whom I am preparing a new and complete translation, but in this edition there are also explanatory notes added to the text.
The text has been prepared in various formats (htm, pdf, epub and mobi), and is available as a Text and Translation and also in an English Only version. The latter also having a flipbook and a reading in mp3 format, which owing to the theme-and-variation like structure of the text is quite mesmerising as it weaves its way through the various permutations that occur.