I've seen the book Otzrot Hayyim recommended as the best first text to use when starting to learn the system of the Ari. When was this book published and where did it come from? References to Kitvei Ha-Ari in seforim usually cite Eitz Hayyim or Pri Eitz Hayyim. Wikipedia says that an additional set of books called the Shmoneh She'arim were published relatively recently, after previously circulating only in manuscript and only in Eretz Yisrael. But Otzrot Hayyim does not seem to be one of the works that made up the Shemoneh She'arim.
The Arizal hardly ever wrote at all (however, the Cairo Geniza has recently surfaced several piyutim that he wrote while living in Egypt). Thus, his teachings come to us from the students who learned from him during his short time in Tzfat. Most of these teachings were written by his closest disciple, R. Chaim Vital. There are, however, books by other students such as Sefer HaKavanot (R. Trinko), Kanfei Yonah ( R. Menachem Azariah of Fano), and Emek HaMelech (R. Seruk).
The most important, and perhaps most complex, of R. Vital's works are the Etz Chaim books (Etz Chaim, Pri Etz Chaim, and Nof Etz Chaim books). Much of the work deals with kavanot and secrets for mitzvot, as well as laying down the foundations of Lurianic kabbalah.
The Shmoneh Shearim are the "eight gates" that are considered less complicated, often go over the Etz Chaim topics, but also range into Tanakh, Gemarah, meditation, Zohar, and reincarnation (they are Shaar Hagilgulim, Hakavanot, Hahakdamot, Hamitzvot, Hapsukim, Maamarei Rashbi, Maamarei Rashbi, and Shaar Ruach Hakodesh). They are each separate works, but generally stick to R. Vital's teachings.
Otzarot Chaim is from another category: those edited by R. Yaakov Tzemach (he was R. Meir Pepperos's student, who edited the Etz Chaim books). It incorporates other students of the Arizal besides R. Vital in framing the system of kabbalah.
The best first texts to approach the Arizal, are the Etz Chaim, Shaar Hahakdamot, and Mavo Shearim (also ed. by R. Tzemach). Etz Chaim is really the foundation of foundations on the Arizal, and according to R. Aryeh Kaplan, "for one who has mastered the contents of this work, the rest is essentially revealed." Shaar Hahakdamot repeats many of themes of Etz Chaim, as does Mavo Shearim (which does mean Introduction to the Gates). Thus, those 3 works are usually the first works studied.
One last note; the very name of kabbalah indicates that such learning need be taught by those capable and fluent in them. A reliable, smicha rabbi is called to the task for one proven to have already filled his stomach with the bread and meat of Jewish sustenance: Tanakh, Mishnah, Gemarah, Halachah, Rishonim and Achronim. B'Hatzlacha!
From getting more familiar with the subject, I've learned that the book Etz Chaim, the most important and most popular book of the teachings of the Ari, was put together by editors from several writings left by Rabbi Chaim Vital. One of the writings was Otzrot Chaim. Whenever in Etz Chaim a passage is marked 'mahadurah tinyana' it is from Otzrot Chaim. These passages are interspersed throughout Etz Chaim (along with passages from the other sources, which are marked 'mahadura kama' and 'mahadura batra'). So, if you have Etz Chaim, you already have Otzrot Chaim, just broken up and interspersed among passages from other sources. But Otzrot Chaim is also available as a separate book and in this form it has been recommended as a text for people first approaching the Ari's teachings because it is less complex and less discursive than Etz Chaim.
Otzrot Hayyim was written by Rabbi Chaim Vital Zatzal, the foremost disciple of the Arizal.