You can find my academic writings in the journal Kabbalah, and in free PDF form on Academia.edu (just search for Jeff Amshalem). There are lots of other great papers there, too, especially if you can read academic Hebrew (if you can, check out the cutting edge research of my advisor, Jonatan Meir).
For less academic resources, here are a few places to start:
Menahem Kallus: Pillar of Prayer (Amud haTefillah): Teachings of Contemplative Guidance in Prayer, Sacred Study, and the Spiritual Life from the Baal Shem Tov and his Circle
Martin Buber, Tales of the Hasidim: Buber has been criticized for recreating Hasidism in his own image, presenting a too-user-friendly picture, one of “Hasids without mitzvas,” as one of my teachers put it. I agree, but they’re still a fabulous resource. Abraham Joshua Heschel (see below) himself suggested starting with Buber. If you combine the tales with actual learning from Hasidic sforim I think you get a nice rounded picture. Then read Hasidism and Modern Man and My Way to Hasidism.
Abraham Joshua Heschel: does he really need an introduction? Ok, just in case, he was a scion of several Hasidic dynasties (Apt, Novominsk, Sadagora, Rizhyn, Berditchev…), a descendant of Avraham Yehoshua Heschel, Shlomo of Karlin, Levi Yitzhak of Berditchev, and the Maggid of Mezeritch, who left his future position as a rebbe to study philosophy and write poetry. A free-thinking writer and theologian with a first class secular education, he took his deep insights into Hasidism and religion in general and reformulated them in a powerful way for moderns. In America he led interfaith dialogue, was active in the anti-segregation and anti-war movement, and connected American Jewry with the Hasidic heritage. His whole world view comes out of Hasidism, even when he never mentions a single rebbe or text. I suggest reading everything he’s ever written but I’m listing the works most relevant to Hasidism.
The Earth is the Lord’s: The Inner World of the Jew in Eastern Europe
Heavenly Torah as Refracted Through the Generations
A Passion for Truth The Circle of the Baal Shem Tov
Arthur Green: contemporary theologian and leading scholar of Kabbalah and Hasidism. Books I’ve read and recommend:
The Language of Truth: The Torah Commentary of Sefat Emet
Menahem Nahum of Chernobyl: Upright Practices/Light of the Eyes
Tormented Master: The Life and Spiritual Quest of Rabbi Nahman of Bratslav
Your Word is Fire: The Hasidic Masters on Contemplative Prayer
Devotion and Commandment: The Faith of Abraham in the Hasidic Imagination
Nehemia Polen, Holy Fire: the story and selected teachings of the Esh Kodesh, Kalonymus Kalman Shapira of Warsaw; he’s also an amazing teacher, so if you live in the Boston area take one of his classes on Hasidism at Hebrew College, or come to one of his teachings with the Beit David Talner Synagogue on Corey Ave/Cummings Rd. in Brighton.
Elie Wiesel: Souls on Fire and Somewhere a Master: portraits of Hasidic masters from a gifted writer raised in a Wiznitzer family
Pesach Schindler: Hasidic Responses to the Holocaust in Light of Hasidic Thought I learned with Reb Schindler in Jerusalem and he is a true joy and an embodiment of Hasidic ideals. I have yet to read this book but if his writing is anything like his teaching it’s worth finding.
Weekly teachings and stories from Rabbi Levi Cooper: http://www.pardes.org.il/online_learning/hassidic-lore/
Collected Rabbi Shlomo Carlebach divrei torah: http://www.shlomoyeshiva.org/reb-shlomo-torahs/