The Book of the Factors of Clinging to Existence is the third of the five books of the Interwoven Discourses. It is named after its first saṃyutta which is the dominant one both in length and importance, since it deals with a key aspect of the teachings. 
 The analysis of the factors of clinging to existence represents the logical level systematization of experience and is key to understanding suffering and describes how we are chained to Samsara. Experience involves five processes: qualia, emotional reaction, perception, conditional situation and cognition. The first is the process related to sense inputs and the other four are the processes of conceptualization. 
Qualia are the result of encoding and processing inputs from the six sense gates: sight, hearing, smell, taste, touch and intellect, and deliver images, sounds, smells, tastes, tactile sensations and ideas. The emotional reaction is the first thing that intervenes through the limbic system and is the process that translates the experience into pleasant, unpleasant or indifferent. A pleasant experience will incite clinging and an unpleasant one, aversion. Perception is the process that gives meaning to the experience. 
The conditional situation affects the experience from the moment in which the same experience is not processed in the same way over time depending on the previous history. Each experience will change the conditional situation so that no experience will be repeated. Our perception of time is a conceptualization of the conditional situation. And finally, cognition is the act of knowing. As we see, all these components are perishable and last as long as the experience lasts. We have no other connection with Samsara than these five elements which, grouped together, we will call qualia and conceptualization. Understand that all five are conditioned and therefore perishable. That what is conditioned we have no control over. And that over which one has no control cannot be said to be "me" or "mine". And that which is neither me, nor mine, which is perishable and conditioned, is unsatisfactory. And that which is unsatisfactory becomes easy to abandon. And abandoning it, one abandons the enchainment to Samsara. 
We see that the consciousness is clinging to existence by these five clinging factors. Liberation is untying the consciousness from these five factors and constitutes the ultimate goal. A consciousness untied from Samsara is a free consciousness and freedom is Nibbāna. 
This is the core of the Dhamma. 
Of the remaining booklets, three continue with this theme and the rest deal with various secondary subjects, some organized by theme, others by persons. Of note is the saṃyutta devoted to the Nāgas, beings whose bodies are described as those of serpents, although they can assume human form at will. Also to the Supaṇṇas, mythical creatures imagined as winged, considered enemies of the nāgas, the Gandhabbas, a class of semi-divine beings who inhabit the Cātummahārājika realm and are the lowest among the devas, and the Valāhakas who are the cloud devas who control the weather.