Inner Space from Rabbi Aryeh Kaplan (written by his students based on his teachings) is a great intro in English. He goes through the concept of ein sof, tzimtzum, the four worlds, the sefirot, partzufim etc.
Another great Sefer is Nefesh Hatzimtzum, an English translation of Nefesh Hachaim (from Rabbi Chaim Volozhin) by Rabbi Avinoam Frankel. There are extended footnotes and a second volume that gives English translation to a lot of the Kabbalah that Nefesh Hachaim is based on.
Finally there is the Sefer שפתי חן (I forget the author) which is specifically written as an intro to the Kabbalah that chassidus is based on.