AZR was among the first to recognize the greatness of Rav Kook (who came to serve in Jaffa in 1904; AZR himself immigrated in 1905/6), and he drew the circle of writers close to the Rav, bringing them to the "third Sabbath meal" (se'udah shlishit) at the Rav's home. The connection also surfaces in his memoirs: in the chapter "A Family Quarrel" (see "The Mother's-Mother Line") he tells of conversing with Rav Kook in Jaffa about the Alter Rebbe's ban, and the Rav answered him with an anecdote from Vilna.
Rav Kook esteemed AZR and entrusted him with manuscripts of his for editing and preparation for print, among them the work "Ein Ayah" (his commentary on the aggadot of the Talmud); AZR handled the manuscript and prepared detailed indexes for it.
Research (documented in scholarship on Rav Kook and in the sources of his school).
Evidence of the closeness: on the title page of "The Writings of A.Z. Rabinovitz," volume 2 (Jaffa, 5674/1914), there is a dedication to Rav Kook signed in AZR's own hand (documented at an auction house); AZR also mentioned "R. S.H. Kook" in his acknowledgments in the introduction to the Bacher translation (see "AZR the Translator").
In 1911 (5671), as editor of the memorial book "Yizkor" for the fallen Hebrew guards and workers (Jaffa, 5673), AZR turned to Rav Kook and asked him to write words of eulogy. The Rav's reply — a rare document of a man of halakhah eulogizing secular pioneers who fell in defense of the Land — was later published as an essay:
"Honored writer, Mr. Z. Rabinovitz, you have set before me a small question…" Opening of Rav Kook's essay "Al Bamotenu Halalim" ("Upon Our High Places Lie the Slain"), addressed personally to AZR, in reply to his request. (In Ma'amarei HaRe'iyah; and in the journal "Sinai," vol. 17, 5705/1945.)
This is, then, an essay-letter by Rav Kook addressed directly to AZR, in which the Rav wrestles with the question of how one mourns young people who gave their lives for the Land yet were far from religion. His answer is among the foundational documents of his teaching on the holiness of the entire nation.
In this essay (written at AZR's request) Rav Kook lays out his inner struggles and the "tragic war of the spirit" in which he was caught: how does one eulogize heroes who gave their lives for the Land but were far from religion. It is one of the founding documents of the Torah-and-Labor synthesis — and it was born of AZR's approach to him.
Research (Rav Kook, "Al Bamotenu Halalim," in Ma'amarei HaRe'iyah; documented as written at AZR's request for the "Yizkor" book).
This accords with his broader work: AZR edited "Yizkor" (which also included a portrait of R. Yehoshua Barzilai), and it was he who cited the "Yizkor" book in his essay "Kinyan HaMoledet" ("Acquiring the Homeland") as evidence of love of homeland (see "Zionism and Building the Land").
In Tammuz 5680 Rav Kook published the pamphlet "HaTechiyah HaShlemah o Techiyat HaKodesh" ("The Complete Revival, or the Revival of the Holy"). AZR replied with an essay bearing the same name — a primary document of direct dialogue between the two. He agrees with Kook that a merely "secular revival" is not enough: Primary source, AZR
"Our master Rav A.I. Kook has published a pamphlet, 'The Complete Revival, or the Revival of the Holy'… He wishes that the holy too shall live — or, more precisely, that the holy shall give life to the secular, refine it and elevate it. I will not conceal that I too desire this, and so I shall try to seek out the way…" AZR, "The Complete Revival" (pamphlet, Tammuz 5680/1920), Ben-Yehuda read/43904.
AZR's path to holiness runs through honest labor: in the Diaspora, life was "founded on fraud and deceit," and if the "filth of exile" persists in the Land, "what revival of the holy can there be?" The remedy: "to base our lives on honest labor… 'When thou eatest the labour of thine hands'… and then… there will be room for holiness to dwell among us." And within the essay he preserves a saying he heard from the Rav himself:
"A saying I heard from the mouth of Rav A.I. Kook mentioned above: 'The boldest of the nations is the people of Israel! In days of old, when mighty idol-worshipping kingdoms surrounded the Land of Israel, a small, poor Levite with shining eyes stood with a small harp on his platform in the Temple, playing and singing with strength and utter certainty: Ashamed be all they that serve graven images… bow down before Him, all ye gods!'" Ibid., Rav Kook's words as AZR heard and quoted them. AZR closes with the address: "Thus, our dear master…".
The connection with the house of Kook was not limited to Rav Kook himself but extended to his son, Rav Tzvi Yehuda: AZR edited with him the journal "HaTarbut HaYisraelit" ("Israelite Culture"), in the spirit of Rav Kook, which even published Rav Kook's "Zer'onim" ("Seeds"). By Rav Tzvi Yehuda's testimony, the collection was intended from the outset to serve as a "counter" to Ahad Ha'am's "HaShiloah," but it ceased after the first issue with the outbreak of World War I.
Already in Rav Kook's lifetime AZR dedicated to him an essay of admiration, "Keter Torah: On the Spiritual Figure of Our Master" (in A.M. Luncz's "Luach Eretz Yisrael," 5672/1912) — an early expression of his esteem for the Rav's spiritual persona. Source "Luach Eretz Yisrael" 5672.