To address your original questions,
1) Sam2 may be right about the book you mentioned, but in general your assumption is correct that one should avoid reading books by non-Orthodox Jews. Even if it is just a translation, there may be heretical material in the introduction.
2) Original kabbalistic sources in general are something to be studied (if at all) when you have more background. However, there’s no harm in learning chassidus on a basic level (some chassidim describe chassidus as kabbalah in a more accessible form), or perusing easy-to-read kabbalistic texts such as Tomer Devorah (which one can access online for free). For example, you could read the commentaries in the (Chabad) Gutnick chumash, peruse the (Chabad) website sichos-in-english, read articles on Chabad.org and study The Essential Rabbi Nachman. Anything by Rabbi Aryeh Kaplan would also be good to read, whether we’re talking about his mystically-oriented works or not. R’ Yitzhok Ginsburg’s website inner.org also contains a lot of interesting and accessible articles on kabbalistic topics.