Reddit mentions of Hasidism: Between Ecstasy and Magic (Suny Series in Judaica: Hermeneutics, Mysticism, and Religion)

I haven't read his book, so I don't know his overarching themes. However, the first few chapters of Universal Aspects of the Kabbalah and Judaism are available online, so I just skimmed them. It seems his views are highly influenced by Hasidism, Sufism, and Advaita Vedanta (Non-Dual Hinduism). I see he's also written some other books, but, of course, having not read them I cannot make any suggestions.

But if non-dual religion is what you're looking for, there just so happens to be a book on non-dual Judaism, Everything Is God: The Radical Path of Nondual Judaism by Jay Michaelson. I've read bits of it, but not the whole book. So I can't offer too much insight. But perhaps it'd be useful for you if this is where your interests lie.

I'm not an expert in Sufism, so I can offer little there. If you're interested in Advaita Vedanta, the Bhagavad Gita holds a lot of influence in Advaita movements. A lot of the Gita can be interpreted in a non-dual way. Plus, in what I read of his book, Schaya quotes from it. So perhaps that might be a book of interest for you. You can find translations of the Bhagavad Gita freely available online. There also should be numerous different translations in any bookstore. Most of my knowledge of Hinduism has come from lectures, not secondary sources. So I'm afraid I don't have any more book suggestions in this sphere.

Schaya also seems to have an appreciation for Buddhism. There's a book I love called The Jew in the Lotus: A Poet's Rediscovery of Jewish Identity in Buddhist India by Rodger Kamenetz. The book chronicles a journey by a Jews delegation to meet with the Dalai Lama. Some of the rabbis involved are influenced by Kabbalah and speak on some of the subjects. There's also discussions on the relation between Judaism and Tibetan Buddhism. Schaya seems willing to incorporate non-Jewish concepts (to a limited degree) into his Kabbalistic views. Kamenetz does not do likewise in his book, instead keeping Judaism and Buddhism respectfully separate. So in The Jew in the Lotus there's no fusion religion being professed, rather a dialogue between two religions. The book isn't directly about Kabbalah, but there are some Kabbalistic undertones.

Kabbalah, of course, is largely derived from the Zohar by either Rabbi Shimon bar Yochai or Moses de León. The Zohar is a massive text, and is largely incomprehensible without someone to teach you. Don't bother buying it. If you'd like to read selections of the Zohar in a clear and understandable way, there's a book I can recommend. Daniel C. Matt is one of the greatest scholars around when it comes to Kabbalah and translation. First and foremost, Matt is a historian. He does not have an agenda, he is not trying to distort Kabbalistic concepts to suit any sort of theology. He is a scholar first. Secondly, he is a Jew. As such, he understand the inextricable link between Kabbalah and Judaism. Matt has a book with selections of the Zohar translated called Zohar: The Book of Enlightenment. It's concise and clear. He also has a book called The Essential Kabbalah: The Heart of Jewish Mysticism which covers a wide variety of topics utilizing well translated primary sources. Here is a video of Daniel C. Matt speaking, just to give you a taste of who he is.

Daniel C. Matt's Zohar: The Book of Enlightenment has a forward by Rabbi Arthur Green. Green is a leader in Reconstructionist Judaism and Neo-Hasidism. He has a book called Radical Judaism: Rethinking God and Tradition which outlines more liberal and panentheistic Judaism deriving from Hasidic Kabbalah. The book is directed to both Jews and non-Jews alike, so it might be a good book for one who is interested in universal aspects and Kabbalah. Green is liberal, but he is still a rabbi and works within the same sphere as Daniel C. Matt; so his book has an agenda but is of a high quality. Green has also edited a few works of Moshe Idel, a renowned scholar in Kabbalah and a student of the father of academic study of Kabbalah, Gershom Scholem.

I don't really know of a good book that gives a scholarly treatment of Hasidism. I'm still actually searching for one. However, I personally like Elie Wiesel's Souls on Fire: Portraits and Legends of the Hasidic Masters. Stories are important in Hasidism, so perhaps the best book on it is not a scholarly treatment, but instead a collection of legends. Regardless, Wiesel's book is engaging and gives one a sense of what the beginnings of Hasidism were all about. There's also Martin Buber's Tales of the Hasidim, but I found it a little drier than Wiesel's book. I've recently acquired Moshe Idel's Hasidism: Between Ecstasy and Magic, but I haven't had a chance to read it yet. Maybe it's good, maybe it isn't.

As always, Gershom Scholem is a great resource. He's not always the most thrilling read, and some of his theories have been questioned by scholars after him, but he's a bedrock in the academic world.