The Suttas are contained in the Sutta Pitaka, within which are 5 collections or nikayas. Of these, the first 4 are the earliest and the ones that can be fully trusted. It is because you will never find any contradictions in it - from one sutta to the other. They are :
- Digha Nikaya consists of 3 books on the long discourses (34 Suttas) of the Supreme Buddha,
- Majjhima Nikaya comprises 3 books containing the Supreme Buddha's middle length discourses (152 Suttas),
- Samyutta Nikaya contains about 2000 short discourses in 5 books, and
- Anguttara Nikaya contains about 2000 short discourses in 5 books.
Khuddaka Nikaya, the fifth, is a minor or smaller collection. Although termed "smaller", it is in fact the largest as more and more books have been added to it over the years. It has grown to 15 books in the Thai and Sri Lankan versions.
In 1956, the Sangha Council in Burma added another 3 books, which are not the Supreme Buddha's own words. These 3 additions are Questions of King Milinda, Petakopadesa and Nettipakarana. This is how the Khuddaka Nikaya grew from a minor collection to become a major collection!
In the future, say in 500 or 1000 years' time, this would definitely create more confusion. Out of the 18 books now, probably only 6 are reliable in that they do not contradict the earliest 4 Nikayas.
These 6 reliable books are the Dhammapada, Sutta Nipata, Theragatha, Therigatha, Itivuttaka and Udana.
As Buddhists, we should be familiar with the Suttas and if possible obtain our own copies. It is a sad fact that whereas we rarely find Muslims without the Quran or Christians without the Bible, yet we find many Buddhists without the Nikayas.