One of the main pillars of Kabbala was the Book of Zohar, from the school of Rabbi Shimon bar Yochai. It was upon the teachings of the Zohar that the two greatest Kabbalists of modern times built their entire systems: Rabbi Moses Cordovero (d. 1570), known as the Rama"k, and Rabbi Isaac Luria (1534-1572), universally referred to as the Ar"i, an acronym for " Elo-hi Rabbi Yitzchak / The G-dly Rabbi Isaac".
The Zohar is opaque. The main relationship between the writings of the Ari (Kitvei Ari) and the Zohar is that without the Ari's teachings the Zohar does not make any sense at all. You could study the Zohar, which is a very poetic text, but you cannot detect any system or structure. Once you know the Kitvei Ari, the entire Zohar becomes an open book.
The main work of the Kitvei Ari is the Etz Chaim ("Tree of Life"). This work expounds the theoretical foundation of the Kabbala. For one who has mastered the contents of this work, the rest is essentially revealed. Then, the Pri Etz Chaim (Fruit of the Tree of Life) and Sha'ar HaKavanot show you how to apply the various teachings of the Etz Chaim to all kinds of daily situations; meditations when you put on tzitzit or tefillin, when you pray or when you eat matzah. They also show how to develop meditative techniques based on the Ari's teachings.
Then come the works known as the Shemona Shearim (Eight Gates). The first gate, Shaar HaHakdamot, (Gate of Introductions), covers the same theoretical ground as the Etz Chaim. The second is Shaar Maamarei Rashbi, the Gate of Zoharic Teachings; the third is Shaar Maamarei Chazal , the Gate of Talmudic Teachings, the fourth is Shaar HaPesukim, the Gate of Biblical Verses; the fifth is Shaar HaMitzvot, the Gate of the Commandments; the sixth is Shaar HaKavanot, the Gate of Meditations; the seventh is Shaar Ruach HaKodesh, the Gate of Divine Inspiration, which is a general recapitulation and describes how to use the Ari's system as a meditative discipline. In many ways, the Shaar Ruach HaKodeshis the key to the entire Kitvei Ari, because all the previous gates deal with theory, while the Shaar Ruach HaKodesh teaches how to put this into practice. The eighth gate is Shaar HaGilgulim, the Gate of Reincarnations.