Gate to the Heart: A Manuel of Contemplative Jewish Practice, is the latest iteration of an essay that Rabbi Zalman Schachter-Shalomi, zl, wrote for the First Jewish Catalog back in the late 60s. As he explains in the introduction of this book, really this is the third or fourth incarnation of the ideas presented in this book.
This small book, at 118 pages of text, is loaded with big ideas and practices to express them. As such, it is a book that must be read slowly, and re-read. Reb Zalman does very little to expand upon the concepts in this book; quite the contrary, he appears to want this work to be cloaked in mystery. Perhaps he is harkening back to the pre-Hasidic days when esoteric knowledge was written in opaque language to fool or tire the uninitiated.
So buy the book, read the book, read it again. Take notes, go slowly, try his exercises. The struggle is worth it.