This is the only book-length study of Rav Zadok available in English and there doesn't seem to be much written about it online, so I'll try and be a bit more thorough than usual. Rav Zadok (1823-1900) was a Jewish thinker of singular importance, developing a remarkably coherent reading of the full spectrum of Rabbinic sources from the central corpus of talmud and responsa through medieval philosophy, kabbala and chasidut. His way of understanding Jewish thought and practice are stunning in their thoroughgoing psychologizing of nearly everything, creating a modern religion of the self--without any obvious points of rupture with Jewish tradition.
Brill's study of Rav Zadok is comprehensive, readable and in many places, very interesting. In others, it can be a bit hard to get through. Ideally there would be a whole library of monographs within which to rank this book. For the time being though, it's one of one. Unfortunately, this book is out of print and wildly overpriced online, so you're going to have to dig around to find it. I read it via inter-library loan from a US university library.
Section-by-section review:
The first three sections (Introduction, From Talmudic Scholar to Hasidic Rebbe, R. Zadok and the Intellectual Mystical Tradition) provide background about Rav Zadok's life and thought. I would recommend them to even a casually interested reader. Highlights include the wild story of his child marriage and divorce, his transformation from Lithuanian style lamdan to Polish Chasid and an overview of how his work fits into his Jewish historical context.
The next three sections (Purity and Desire/Dreams and the Unconscious, Determinism and Sin: Izbica and His Antecedents, Determinism and Sin in R. Zadok's Thought) discuss the most striking parts of Rav Zadok's thought and situate them within their historical context. Here we encounter the central role of desire in R. Zadok's understanding of man's telos according to Judaism, the struggle with the unconscious and the fascinating history of yerida l'tzorech aliya and aveira lishma from the Shla, Izbica and finally R. Zadok. Brill also mentions how similar ideas play foundational roles in Sabbatian thought, which was new to me and gave me better appreciation of that movement.
The next three sections (Religious Growth: Mitzvot, Prayer, Repentance, and the Holiday Cycle, Mystical Divine Unity, Torah Study as Mysticism) get pretty dry. Here, Brill explicates R. Zadok's views on the title subjects and connects them to the movements which came before him, showing how he synthesized Gra/Volozhin and Chabad/Izbica systems into one. It's strange to read these sections because, to me at least, what is so fresh and powerful about R. Zadok's approach is that he either eschews or subtly reinterprets the conventional views on Torah Mitzvot and the nature of the Godhead. Maybe I just don't understand the context well enough to appreciate it. In any event these were hard chapters to get through, reading too much like theosophy to make sense to me.
The final three sections (Text Language and Experience, History and Torah, Modernity) are more interesting. Here Brill highlights key innovations in R. Zadok's thought compared to what came before him on subjects that are extremely modern: textuality and history of religion. Of course, like everything else his understanding of modern problems is anything but modern, and yet it deals with the problems that arise as the individual becomes more aware of historicity and of himself as a "self". The final chapter (Modernity) stands alone and discusses, mostly in general terms, how R. Zadok's works answered specifically modern concerns that arose in his day (urbanization, shifting authority to the individual) and that continue to be relevant.
The two appendices (R. Zadok's Works and Intellectual Mysticism) are excellent as well. None of R. Zadok's works were published in his lifetime, though he wrote them from age 13(!) until his 70s while undergoing several significant personal transitions. Brill's brief discussion of their chronology and publication history is invaluable to any student, trying to orient his or herself in the confusing sea of books that have been published. The appendix on Intellectual Mysticism is similarly enlightening, describing how Aquinas' position on the unknowability of the Divine without Grace cast a giant shadow over the study of mysticism, casting out "intellectual mysticism" (where the mystical experience requires and/or supplies actual knowledge) from the study of mysticism. Effectively that means a priori declaring Islamic, Eastern Christian and Jewish mystical thought as "not mystical." This mistake has been slow to remedy and the full ramifications of an encompassing understanding of mystical thought remain to be seen (at least as of 2002 when this book was written).
In summary, I would recommend the first three chapters and the last three chapters of this book to anyone who is interested in understanding how traditional rabbinic Judaism can be understood in light of modern consciousness of time and self. Students of R. Zadok who want to better understand his thought should definitely read the whole thing. Good luck finding a copy!