Lessons from Shlavei HaSulam (Rungs of the Ladder), by Rabbi Baruch Shalom Halevi Ashlag (the Rabash), which can be read here:
http://www.kabbalah.info/eng/content/view/full/63231
It's recommended to read Shamati as introductory material first, here:
http://www.kabbalah.info/eng/content/view/full/58016