Kaiten Nukariya - The Religion of the Samurai: A Study of Zen Philosophy and Discipline in China and Japan (1913)
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The author claims this is the first book on Zen in English and as far as I can find that does appear to be true. Even so it's not much of a beginner's guide since it goes into quite a bit of detail on both history and practice. (There's a two-page footnote right at the start.) The view that Zen was a samurai religion seems a bit odd today and was questioned not only by some recent work (Oleg Benesch's Inventing the Way of the Samurai, Thomas David DuBois' Religion and the Making of Modern East Asia) but even in the contemporary review in The Nation (April 2, 1914). (The Nation's unnamed reviewer also chides Nukariya for overlooking an "apposite" Tennyson poem though overall the review is both positive and informed on the subject.) The recent writers point out Nukariya's nationalism and his attempt to downplay Zen's ritualistic, religious side.
Nukariya (1867-1934) was a professor at Komazawa University in Tokyo and a friend of D.T. Suzuki (who according to one source wrote the second English-language book on Zen in 1927). Nukariya apparently wrote this book while living at Harvard.